My planes generally retire themselves before I'm given the option... As long as they're fun to fly and can be repaired I generally keep them in the rotation.

I've only retired 2 planes over the years. One is a 1/2a Zero that turned out to be too much work to fly due to a high wing loading. The other is a Stik-clone that already had so much reglue and recovering that there was nothing left to glue to that had any structural integrity. Both are hanging from my son's ceiling.

EG