If you consider the firearm to be the firearm - i.e. the receiver - and not just the parts, then the life becomes nearly infinite for most models. A guy can burn out barrels, burn up triggers & springs, burn up recoil springs, etc, but wearing out the receiver itself is a very rare and difficult thing. Some pistol frames do have a history of cracking, and guys have experienced failures at the extension union in AR lowers, but overall, you can rebuild the extraneous parts many times over before the actual receiver itself even starts to break a sweat. And of course - depending upon the wear to the receiver itself, you might be talking about a simple warranty/OEM replacement. For example, I have an S&W 329PD which has been replaced once and repaired twice otherwise by S&W because of top strap cutting to the sacrificial blast shield. They're fine with it, I'm fine with it - the part is consumable, and if I shoot it too long before sending it in, then the revolver is consumable... Equally, I found a Ruger Vaquero which had been shot loose by a CAS shooter for a very good deal - sent it back to Ruger and they replaced it... "Normal" wear and tear is a very subjective timeline, meaning a revolver shouldn't "normally" wear out in the lifetime of the owner, so when they do, the OEM makes it right.
Most people, however, don't replace their firearms when the receiver itself needs replaced, but rather do so some time long before that. Most guys won't spend $500 to rebarrel their $500 deer rifle when their groups start opening up, they trade it in and buy a new rifle. I have rifles on their 6th or 7th barrels, and I know guys who have a lot more barrels on their rifles than mine.