It's hard to say, I still clearly remember monolougues , and portions of some shows I did in High School, and I graduated 10 years ago. I think it depends on how much the show or character means to the actor.
I'm also coming out of semi-retirement as a magician for a show next week and I was surprised how much of my old kid-show routines came right back to me when I started rehearsing.
Some people come into our lives and quietly go, others stay a while, and leave footprints on our heart, and we are never the same.