^More fun than clay... Not in my opinion. Used to play on clay, now on hard courts, and clay is much funnier IMO. The sliding, the drop shots, I really find it fun when it's well maintained. And clay is pretty terrible on balls too.
Personally, I switch when my knees say "no more", or when the outer heel is bare. I'm not that hard on my shoes, but cushioning is important for me and when it's gone I change shoes. I waited for too long with my previous pair, and sometimes my knees recall me of it. Next time it'll be Yonex I guess.