I don't think of the "Choice" a Changeling has to become a full Fae is usually all that much of a true choice most of the time. I imagine the Changeling feels the constant pull of the supernatural, knowing that there's power for the taking there, if only he accepts it. And accepting a little bit at a time is easy and feels safe ... and besides, he can give it up at any time, right? But while there are those, like Meryl, who make a conscious decision to "go all the way" (and gain the full potential of that power that's been beckoning them, all at once), I suspect that in most cases, the Changeling just continues to take a little bit more power, then a little more, until there comes a day when he realizes that he's gone too far and that there's no longer the option of Choosing mortality.
Mechanically, either of the above options is easy to manage. If the character is an Ogre Changeling, for example, then he can slowly accumulate powers from the Ogre template. If the character makes the "go all the way" Choice, then he gets the full list added to his sheet automatically. This takes no time, as RevengeofTim points out. Alternatively, he can add powers one at a time ... and once he gets to that -10 limit, he's "gone too far".
On the other end of the spectrum, until the Changeling has reached that point of no return, then they can make the Choice to give it all up. There isn't a mechanic for this, but I think the implication is that it's just as quick. I would say that using accumulated Debt compels to ... 'encourage' the Changeling to delay such a Choice would be very reasonable, so basically the Changeling would need to accumulate at least as much Fate as he had Debt before making this Choice is even possible.
Once the Choice was made, the effect is as Richard noted: access to Changeling powers disappears permanently, and the character is now purely human -- or more precisely, Pure Human. The character's powers are removed, his High Concept should be altered accordingly, and as a Pure Human, he would gain the Pure Human bonus.