During the 18th century, British Ordnance considered the "normal" lifespan of an Ordnance issued "King's" smoothbore Musket to be 10-12 years and that included either Peace Time or War. This even though in Garrison in either peace time or war when they were not fighting or campaigning, the Soldiers stood an inspection with their arms every day and the barrels and locks had to be free of rust and properly oiled and the brass polished and the stock finish in good condition.
Looking at the lists of repair parts and tools that British Ordnance sent along to the colonies with the Regimental Artificers/Armorers for both the FIW and AWI, it seems apparent they were kept in top maintenance condition as well. However, these arms were most likely used much more than many civilian arms.
A smoothbore used only for recreational hunting could have lasted a very long time, if they were cleaned and preserved correctly by town or village folk. A smoothbore that was used on the frontier, would have been used more and thus would naturally wear out faster.
Of course the better quality guns with better refined Iron used in making the locks and barrels, would also have lasted longer than less expensive Iron used in some to many Trade guns.
So it would be really hard to say how long a civilian smoothbore would last in the 18th century because it would depend on the quality of the gun to begin with, how often it was used and how well it was cleaned and maintained.
We "modern folk" often have unusual views of how long guns do or should last. We might look at our Grandfather's or even Great Grandfather's Model 70 rifle and are amazed by how accurate it is when it may be 50 to 70 years old, or even older. However, since most folks don't shoot a box of 20 rounds a year, the barrels are just "broken in" and have at least that many more years of use (if not double those years) left in the barrels. The actions are good for at least four or five or barrels or more, when properly cared for.
Yet someone who often competes in high power competition will burn out a barrel in anywhere from one to three years, because they shoot them so much.
So even modern guns will "wear out" pretty soon if they are shot a lot and especially if they are not properly cleaned and maintained.
Gus