the virtuosos usually don't need a lot of practice in order to perform. we have to consider that most of them had learned the basic repertoire when they were still young, played those pieces numerously all their concert lives, and so everything is in their head... they just need to review them once in a while.
the young ones i would guess practice more, although we also have the gifted ones who dont practice that much too. the young ones build repertoire and building repertoire means not only learning as much music but also performing those works in public often. everytime one performs a certain piece in public, it grows in a pianist. it becomes almost like he breathes it, it's in his system.
but basically, everything's in their head: those passages, chord progressions, fingerings, leaps, technical stuffs, etc. when they get back to it, it's easy.