I too have been using the Gatorskins and like them quite a bit. I ride on some rough roads and when I skid I am often skidding through rough debris. I always am afraid they'll puncture but they never do. I've been riding/skidding on them daily since August and they are finally starting to show a bit of square off. I'm not exactly sure when I should replace, but probably soon.
I will say that now that I've worn them a bit, when its wet out they will slip and slide very easily when skidding, so be careful.