Last a long time if properly applied. Myself, they have lasted 15+ years and going strong. From a bowyer that can install them on the form while building them? Probably forever!
All depends on finish, I use a quality mink oil blend (montana pitch blend is great)instead of a sprayed on finish. I oil them whenever I do my boots and leather goods and all is great and happy in the world. If they begin to dry out and peel at the edges, a bit of super glue will cure that quick. Then go back to oiling.
I'm a snakeskin junkie and have applied them to just over 100 bows now. As Shaun said, snakes are quite a bit different from one species to another...some thin and fragile, some like applying slabs of leather. But hey, its another whole avenue to explore and dig into in trad archery. Fun! I've purchased snakes that have died from snake sellers and pet shops. Skin them, tan or dry and you can have a bow that most guys have never seen. I had to get a class 6 dealers license here in Ma. to buy them...but worth it. If you mess up, hey, I guess you are just going to have to put another cool looking set on!