Much as I hate fiberglass (much prefer metal) I suspect there is no pratical life span as such if it is properly maintained. One of the great things about fiberglass is that it is almost perpetually fixable. Take a boat with a really bad case of blisters. You could in fact plane off virtually the entire hull below the waterline and essentially use what is left as the mold to layup a whole new hull on top of it. By using newer, stronger, and more water resistant materials it could easily be far stroger than it was was when new.
My Etap 26 is a pretty good case in point. Due to TCDA (Terminal Case of Dumb Ass) on the part of the original designer the hull has deformed one full inch in the area around the keel trunk. That took 25 years though, and the bottom of the hull is no longer as flat, which actually makes the shape better at taking the load. I took out almost nothing, and added a massive amount of epoxy fiberglass reinforcing on the inside of hull. So I have no doubt that the hull is now stronger than it was new.
By the way this is my second hull rebuild, I did a Hunter 27 to far better than factory strength about 25 years ago. I also stregthened the rudder support, and the propellor strut area, with very little effort.
Gary H. Lucas