Barrel life means different things to different people. Three common measures are:
1) To a precision shooter, when groups increase noticeably, perhaps 25% from their best averages. So say a barrel shoots .75 MOA consistently with a handload recipe, and starts doing no better than 1 MOA. To the F-class or precision shooter the barrel may need replacing. Most casual shooters wouldn't even notice.
2) To the military, when muzzle velocity has decreased 200fps from the standard for a new barrel. This is pretty dramatic and chances it won't be accurate at that point either.
3) To anyone who ever shoots on paper, when bullets start keyholing. By that point accuracy is probably "minute of barn" or thereabouts.
With semiauto and privately purchased ammo, very few people will ever reach the 2nd and 3rd measures above. And most of those that do will be high round count users running Wolf or other bimetal jacketed bullets while getting barrels very, very hot.
With copper jacketed bullets and reasonable firing schedules (not mag dumps), even unlined, untreated barrels are likely to last 10-15,000 rounds, carbon or stainless steel. Chrome lined and nitride barrels will probably last 2-3 times longer, or more.
I've been watching for years to see anything definitive on nitride vs. chrome lining and still have not. The military did some testing in the 1960's and at that time chrome lining was definitely better for machinegun purposes. That may not be correct for slow fire and it may not be the same with modern treatment methods. My hunch is that nitride is probably a little less durable than chrome lining, but I haven't seen any hard evidence proving it either way.
Very few people will wear out a nitride or chrome lined barrel with copper jacketed bullets, even for competition purposes.