Unless 10's upon 10's of thousands of shots are being run through a barrel, a casual muzzleloading enthusiast certainly won't wear out a barrel from typical shooting volumes...as a reference, I have a fairly new TC .54cal RB barrel, have shot about 1500 patched balls through it in the past 3 years, and the bore's hardly broken in, still looks like it just came off the production line, still has bluing inside the muzzle, etc
I'd tend to agree with this assessment. Black powder and subs just don't produce the heat and pressure necessary to erode even the relatively soft steel used for ML barrels. And patched lead roundballs, and lead conicals and plastic sabots for that matter, aren't going to produce any appreciable wear in many times the number of shots anyone is ever likely to fire through the gun. I would guess that 50K rounds or many more would not be unreasonable for total barrel life.
The three things that will kill a ML barrel are improper loading/cleaning techniques and equipment that round off the lands or score the bore, insufficient cleaning resulting in plastic or lead fouling buildup, and lastly, a bore that's not promptly and sufficiently cleaned and preserved, resulting in rusting and pitting in the bore. Black powder and Pyrodex residue are both very hygroscopic (they attract moisture from the air) and when exposed to moisture in even small amounts creates a very corrosive mixture that can severly rust a bore in a matter of hours. The best way to prevent this is to clean the gun ASAP with hot soapy water to flush out the residue completely, followed by a thoroughly drying and preserving the bore with a good preservative (I use CLP, very sparingly. A little goes a long way. Just run a dry patch/mop down the bore and snap a couple caps/primers before loading to clear any residue before loading). Other than those things, if properly cared for your barrel should last longer than your lifetime. If it's fouled with lead, give it a good cleaning like Cayugad said and it should be good as new.
Mike