Not exactly a ski, but I have a kevlar skin coat (no gelocat) north american open canoe from 1987 thats good as new, I had a different one from 1983 that was just slightly brittle, but still very serviceable.
I also had a V10 from 06' for a little while that wasnt showing any signs of dying.
Resin is good for a long time. The main determining factor is probably abuse (rocks, drops, surf, garage roofs, gale force winds while driving, excessively tight straps, ect). I'd say human inflicted damage is what ends a boats life much more so than anything else.
Currently paddling a Kai Wa'a Vega Flex in Southern California's ocean waters
Past Boats: Epic V10g1, Stellar SRg1, Fenn XTg1, Swordfish S
"When you've done something right, they wont know you've done anything at all"