It depends on what the saddle is covered with. It also depends on what you wear when you are sitting on the saddle. Jeans will wear out a saddle a lot faster than bike shorts will.
I never heard of the gel breaking down.
I have one with that black covering that looks like spandex it has about 5,000 miles on it and the cover is just now starting to come apart. The gel is fine, its about 10 years old. I have two with just a little gel padding and real leather covering. One has 8,500 miles the other has about 6,500 miles on it, but they are not as old in years. They look almost new.