Yesterday I ran through Seisan kata several times - for the first time in 15-20 years! and it's still in there!
During that time period, I've spent most of my karate kata time on Sanchin, Tekki, and Heian. I haven't done Seisan regularly since about 1990, but it's still all there.
I guess if that muscle memory is going to persist for 20 years or more, that it's time to dust it off, knock the rust off, and begin digging into it for real along the lines of Kane & Wilder.