Most of us cannot keep all the pieces we have ever learned at concert standard at all times. However we know how to "brush up" pieces we learned before and it will take much much less time than what it took to learn it initially to bring it back up to standard. You should if you want to perform in public, be able to maintain at least 1+ hours of playing at concert standard.
Maintain and upkeep old pieces you have learned. That might require you to at least sight read through the piece at least once every few months (even if you think you have it memorized). SIght reading the piece keeps you more consciously aware of what you are playing and take away a little from the muscular memory. Relying on muscular memory for memory of passages is good but sometimes it changes (the music, fingering, technique etc) without us realizing it, we should upkeep pieces with sight reading to prevent these errors from arising.