I don't know enough to answer that precisely, unfortunately. I do know that professional racers replace their bikes something like every season-- once every year. So everybody buys carbon to be just like the professionals, but only the professionals can afford to replace those bikes every year. Of course, the reason they're replacing them every year is because they ride a lot of miles during the year, which is what creates the stress fractures.

Hopefully, somebody can give you a better answer than I can about the lifespan of carbon and aluminum frames. I know there are people on this forum who know the answer; it's just a matter of them seeing this post.