I am quite new to Blackpowder muzzleloaders, but have some experience in high powered rifle shooting.
The things that wears out barrels, while actually shooting, is velocity, friction and heat.
Strikes me that black powder muzzle loaders does not generate much of any of the above mentioned.
I think when it comes down to wear on a muzzleloader barrel, it would all be from the loading rod, and wear from the patching material.
The soft lead and lower pressures would not wear on the barrel much at all.
IMHO, careful use of the rod and careful cleaning, the bore should last a very very long time and many rounds down range.