Once the Fidelius Charm is in place, that which is being hidden will remain so almost indefinitely. It can be dispelled, but as with the revelation of the information contained within, and indeed with establishing the charm in the first place, the process is difficult and must be done willingly by the Secret Keeper.
The exact nature of the method to dispel the Fidelius Charm varies based on the methods used to establish the charm.
It is commonly believed that the secret hidden by the Fidelius Charm is stored in "the soul" of the Secret Keeper. This is a simplified view which doesn't exactly describe how it works — not to mention the ongoing arguments as to the nature of or existence of the soul.
— Excepts from On The Fidelius Charm, from Meditations And Essays On Advanced Charmwork
Flitwick looked around the circle of people filling the drawing room of number twelve, Grimmauld Place. Ron had made a lot of progress getting it cleaned up, and all of the furniture had been removed. There was now just a single small table, on which sat a tiny, solid gold cauldron, with its own small fire underneath. A small silver knife sat next to it, along with a box of potion ingredients.
"Now, this is going to be one of the most complicated spells most of you have been part of." He smiled. "Which should make it rather fun."
Around the room, there were as many members of the old Order of the Phoenix as could be mustered. Several of them, Harry especially, looked rather dubious about the spell being fun, especially with the knife there.
"You all became Secret Keepers for this house on Dumbledore's death. That means, in order to dispel the Fidelius Charm on the house, we need your involvement."
Harry raised his hand, having been so used to being in class. "Professor, we've not got all the Secret Keepers here..."
Flitwick nodded. "Quite so, Mister Potter. The Fidelius Charm is a complicated and, to be frank, poorly understood piece of magic. As this particular secret isn't being kept within a single person, the magic is somewhat... diluted, shall we say?" He shrugged. "That's not to say that having an incomplete group is going to make this easy, but then it was never going to be easy."
He stepped forward to the table, which had been set low enough that he could work at it (since he was going to be doing most of the work for the spell). Everyone else would just have to deal with the height difference, as he usually did. He looked around at the assembled old warriors (not that most of them were old, but the marks of the war were still visible) and asked, "Now, has anyone performed this spell before?"
Arthur and Bill Weasley both raised their hands. They had been looking less unsure, but no less nervous, than everyone else.
"Ah, excellent." Flitwick smiled, then looked around again. "Nothing like a good teaching opportunity for the rest of you, eh?" He opened the box of ingredients and components. "So, this is something like a charm, but given the nature of the magic, involves something resembling potions work as well. Miss Granger, if you'd assist me? I've heard many things about your steady hands at the cauldron."
Hermione looked confused at that.
"You do realise that your teachers at Hogwarts would discuss the students, yes? Why, given how much of our time is taken up with wrangling you all, if we want to discuss our days at all, it ends up being a discussion of the students."
She nodded, and stepped up to the other side of the low table.
Harry thought about that description of the spell, being somewhere between a charm and a potion. He'd never heard about anything crossing the lines like that during his time at Hogwarts, the different departments of magic were always kept very separate. He'd have thought charms and transfiguration might have had crossovers, but charms and potions?
But then he realised. He had seen this sort of combination before. But it hadn't been at Hogwarts. It had been in a graveyard. Bone of the father. Flesh of the servant. Blood of the enemy.
Was this sort of crossover reserved for stronger magic? Perhaps for older magic? Maybe, in the past, they hadn't had such strong lines between the types of spells, and so combined them more freely. He would have to ask Hermione later.
Flitwick took one of the bottles from the box on the table, and poured out its contents into the cauldron. "We start with pure water, taken from the snowmelt of the first snowfall of winter. Getting rather hard to acquire, given the weather lately, and of course the various things which end up in the snow." Hermione wrinkled her nose at that last part.
He took a second bottle, and at the same time he took a silver rod, marked with several runes down the side, which he offered to Hermione. "As I start adding the mercury, I'd like you to stir. Three turns clockwise, one turn back, then four diagonal strokes. Exact one second rhythms, if you please." He unstoppered the bottle, and as Hermione began to stir the complicated pattern, he slowly dribbled the heavy silver liquid into the cauldron.
Once the bottle was empty, he had Hermione switch to diagonal strokes only to stir, as he retrieved a small pouch of dried leaves from the box of ingredients. He held it up for the others to see, still feeling himself to be in teaching mode. Seeing the label, Hermione asked, "Why does this need dittany? I've only seen that in healing potions before..."
"Well reasoned, Miss Granger! Oh my yes, this is a very unusual inclusion in such a situation, isn't it?" Flitwick grinned at her. "Or is it? Consider, the nature of the magic, sealing a secret inside a person which must be pulled out again, and perhaps causing a scar upon the world where the secret had been. Are we not trying to heal this situation?" He shrugged. "But perhaps there are less poetic reasons. The logic of magic is often rather opaque."
After carefully breaking them in half, one by one, he dropped each of the dittany leaves into the cauldron, carefully timed to the stirring. Next, he waved his wand over the top, whispering an incantation, and the leaves dissolved into the mixture, which slowly turned yellow as the mercury and water started to mix instead of staying distinct.
The final ingredient from the box was another small fabric sachet, which Flitwick extracted before closing the box. As he untied the string, he told Hermione to change her stirring again, this time to a series of quick figure-eights. Another wave of his wand, and the churning potion started to glow faintly. As he gradually sprinkled the mustard seeds into the cauldron, there was a green spark as each one hit the surface.
He had an interesting point he was itching to make to them, about how the use of "eye of newt" in many old recipes had actually been from a time when potion books might be more easily found by muggles, and so they had encoded the ingredients to protect against accidents. Eye of newt had been, in fact, simple mustard seeds, whole. Of course, once that had faded from memory, people had found potions uses for actual eye of newt (much to the newts' dismay). However, the spell was getting to a critical phase, so no time for such distractions. He'd have to remember to tell them later.
Once the last seed had hit the surface, the potion was now turning red. Hermione kept stirring, but after a moment she had to fight the urge to stop (she'd seen what could happen if you stop stirring a potion at the wrong moment). "Professor, I think this rod is starting to dissolve!"
Flitwick looked into the cauldron, and nodded with satisfaction. "Good. Silver is, in fact, the next ingredient. Seven more stirs, and then just drop what's left of it in to dissolve." He stepped back slightly. "Without splashing, of course."
Seven more careful figure-eight stirs, then Hermione lowered what was left of the silver rod into the cauldron, making sure to keep her fingers clear of the liquid. She stepped back, watching curiously to see the next stage in the process.
Stepping forward again, Flitwick picked up the silver knife, turning it over several times in his hands. "The next part of the spell is, I am afraid, somewhat less pleasant. I will require three drops of blood from each Secret Keeper present. And it must be taken by yourselves, willingly. Only a small prick will be required, of course, but I'm afraid we can't do anything to close the cut until everything's complete.
He held his hand over the cauldron, and readied the knife, but then he paused, looking around at them. "Oh, and before we begin with that stage, a word of advice. Whenever people want to be dramatic about drawing blood, which of course they do, they draw the knife across their palm. PLEASE don't do that. A small cut somewhere not loaded with nerves and tendons will make the recovery much easier."
His slightly exasperated advice given, he pressed the knife into the tip of one finger, and waited as three glistening red drops fell into the cauldron. As a fourth started to well up on his fingertip, he pulled it away, letting it drip onto the carpet instead. A frown as he looked down, then shrugged. "I'm afraid we shall have to clean your carpets after this exercise, Mister Potter."
He offered the knife to Hermione, because she was still standing closest. "Miss Granger, would you like to go next?"
She nodded slowly, reaching out for the proffered knife, but jerked her hand away. "Um. Maybe someone else." She stepped back a couple of times, trying to be the furthest from the scene.
Arthur stepped forward, taking the knife. "Well, since I've done it before." He very carefully ran the knife along the side of his hand, well away from anything that might be problematic if damaged. His cut was slightly less minimalist than Flitwick's had been, but he still managed to make sure that exactly three drops fell into the cauldron.
He then handed the knife on to Bill, who had moved up to stand next to him. "Better out than in, I always say." The attempt at levity was a little jarring in the sombre, dramatic environment that the rituals around the spell had been casting over the room.
Each of them added their drops of blood, in turn. Most were careful and economical about the cuts they made, not wanting it to hurt any more than absolutely necessary. Watching the whole thing unfold, Harry had a momentary thought of being glad that Mad-Eye Moody wasn't here. His flair for the dramatic would probably have resulted in slightly more blood than the recipe called for, which Harry couldn't picturing turning out well at all.
All that considered, when it came time for Harry to add his own drops of blood, he put the knife across the palm of his hand without thinking, exactly as Flitwick had been describing as what not to do, and it was only Ginny quickly grabbing the wrist of the hand holding the knife that stopped him from causing some damage. He blinked, looked down at what he'd been about to do, and sighed.
The process took some time, as each of them were trying to be careful, and several of them were quite nervous about the process. The fact that they had to draw the blood themselves, with a knife, and willingly, made the whole thing more of an ordeal. Hermione was already starting to come up with plans for a silver magically-valid syringe, given her experience with the sensible nature of muggle medical advances.
Once everyone around the room had made their... contributions, Flitwick put the knife back on the table, and started to wave his wand over the cauldron again. By now, the mixture inside was a deep burgundy colour that almost seemed black, and bubbling quite vigorously despite the relatively small fire.
"Now then, the last stage. We must extract the secret itself." He placed the tip of his wand against his temple, concentrated for a moment, then pulled it away. Hanging from his wand was the wispy silver filaments of a memory, which he carefully deposited into the cauldron. "Much the same process as a pensieve. But remember, concentrate only on the secret itself, rather than anything else about the house. We wouldn't want to have to start again, hmm?"
Ginny raised her hand. "And for those of us who've never used a pensieve?"
"Ah." Flitwick looked around the room. "Who hasn't had to withdraw a memory like this, before?"
Everyone under the age of twenty-five raised their hand.
"Well then! Gather round, gather round. We'll do a practice run first." He stepped off to one side, then looked at the others. "If you could make your contributions, while I'm showing the way? Thank you."
He pulled a small bowl off one of the nearby shelves. "To produce a physical memory like this, there's no incantation, and the wandwork is purely to place the tip of your wand against your head, then pull it away as the memory begins to come through. It's all in the intent."
Nobody raised any objections to this, so he continued, "So, wands out! I want you to concentrate very clearly on the memory of the secret, of the address of this house. Then, push it to the front of your mind, and towards your wand. When it starts to tingle, pull your wand away, and it should come. Now, try!"
None of them managed to pull the memory out on the first try, but after a few attempts, Ron managed to pull the silvery threads out, which dropped neatly into the bowl that Flitwick was holding up for the purpose.
Flitwick swished it around a little, and poked at it with his wand, then nodded with satisfaction. "Very well done, Mister Weasley. Ten poi—" He stopped, and shook his head. "Sorry, force of habit, that." He held the bowl up to Ron, "If you'd like to take it back on your wand, and deposit it in the cauldron?"
Getting everyone's memories into the cauldron took a little longer than expected, with everyone being careful to get exactly the right memory out, and the fact that most wizards don't get much practice with extracting memories for a pensieve, even when they know the theory.
The worst part of how long it was taking was the distraction from the aching cut marks each of them had on their hands, which Flitwick had stopped them from healing away instantly. It would, apparently, interfere with the spell.
Finally, the tiny gold cauldron was about half full, and as Flitwick started one last series of wand movements and whispered incantations, the mixture inside started to shift into a bright golden colour, glowing enough to light the whole room. Tendrils of translucent ... something ... started to emerge from the cauldron, reaching up towards the ceiling and then spreading outwards.
The filaments seemed to fade slightly as they spread, and were wrapping themselves around every part of the building, going beyond just that one room. Soon they had faded from view entirely, but the feeling of powerful magic being done lingered in the air for nearly a full minute afterward.
The cauldron, now empty, rattled slightly as the feeling faded, and Flitwick nodded in satisfaction. "I do believe we now have a house cleared of the Fidelius charm. It would have been easier with a single Secret Keeper, but we make do." With that, he pulled out his wand again, and started to cast a healing charm on his hand. Like a flash, everyone else took that as permission to deal with their own injuries, and the sound of hastily, eagerly shouted healing charms was followed by quiet sighs of relief.