If you're shooting cast bullets, and using a colder burning powder, I'm not sure you would ever wear a barrel out. I've got a friend that won't use certain powders(especially Blue Dot) because they burn "too hot". He has been shooting the same Marlin 336 since 1970. He says he has never shot a jacketed bullet through it, and has never used anything except a patch and jag to clean the barrel. The bore looks new.
I've not had any trouble with Blue Dot, but I wasn't shooting several thousand rounds a month either. He claims he once got a gun so hot shooting Blue Dot loads it screwed up the heat-treat on the gun. I still have a hard time imagining this, but he hasn't lied about anything else, so I'll take his word for it.