Well, it depends on the fragrance as well as the proper storage. I think a good indication of a fragrance life is it's longevity, makes sense doesn't it? if it evaporates faster on your skin, it does the same in the bottle.
Now, I had fragrances go bad due to bad storage, but it's usually the smell being lighter/different. The longevity on the other hand does not really change significantly, at least from my 2 or 3 bottles that went bad, they just smell different. I assume it is affected by age, but not as much as the perfume strength and actual smell. Keep in mind, some vintage perfumes decades old are being sold for hundreds on ebay, and people swear by them! I doubt anybody would pay $300 for a 1oz of shalimar parfum if it lasts 2 hours. On the other hand, people have complained about creed bottles going bad after a year or two, but hey creeds are mostly fresh fragrances with weak longevity.
So yeah, longevity changes but I'd say it's the least affected attribute. It worries me a little, so does projection, you can always apply more after all. However if the scent changes, to worse, then I'm screwed. And in general, a well packaged fragrance should last you a good 5-10 years. Probably a few decades if you take the extra effort to store it properly. If vintage perfumes are still great after 50 years when I doubt people took the extra effort for proper storage, you have nothing to worry about.