"Well, we would have grown back eventually, but my that would have been a pretty boring century or so" answers Celestia. She looks over at Twilight. "Oh, look, we're almost finished!" You look over and see that Twilight seems to have her brain back entirely, although it still look as though she's sleeping. The glow of Celestia's horn intensifies as she concentrates, until there's a flash, and standing before you is a whole-again Princess Twilight Sparkle, wings extended majestically. She looks upon you and the rest of the girls, as well as the beaming countenance of her mentor.