Ammunition and use are two variables which determine the wear on a firearm. Another important variable is the maintenance the user gives the gun. The cleaner the better as dirt has an abrasive effect on moving parts. Given proper care, a gun will outlast the lifetime of the user. Consider that when the North- South Skirmish Association first started up, they were using original Civil War weapons. Finally, any modification made to the gun which are outside of the original factory specifications could either shorten or help lengthen a gun's life span. Those recoil buffers works wonders for reducing the slamming, but they can adversely affect reliability.
Mind you, some parts are more likely to require replacing than others. Recoil springs or magazines (or parts therein) for pistols. On some revolvers, the hand (or pawl can wear and may need peening or replacement). Same with the cylinder stop (bolt).
Rule of thumb: If you shoot a lot, have your gun inspected annually by a gunsmith. This should entail complete disassembly and cleaning, inspection of parts and replacement of worn parts where necessary.