Ironically, last year i decided to relearn piano after some 40 years. Learned one piece (a pretty good one, fortunately) and a once a day play triggered it. So. It's more like once every 2-3 weeks now, not long enough to forget, but long enough to keep the thumb feeling all right.
This is my biggest apprehension. I've become quite a good guitarist, and I know that can't last forever. Getting my accomplishments in while I can.