Someone in a private message astutely pointed out that this is a very vague question. Let me elaborate a little:
I realize it's a question with many dimensions. However, it is generally true that guns outlast cars, electronics, vacuum cleaners, bicycles—few products have the average lifespan of a firearm, which tend to outlast their owners if they are properly maintained.
To narrow it down a bit: I'm interested in answering this question for all the subsets of firearm, but I'm more interested in the most popular ones, like the Glock and the AR15. What's the life cycle of the average American firearm look like? It leaves the factory, is sold, how many owners does it have, what kind of wear and tear does it get given the level of maintenance practiced by the average owner, and after how many years does it become unusable.