Life span of a wad gun

How many rounds will a well maintained wad gun shooting lead bullets go before something needs attention. What normally is the first thing that needs attention? Is it:... slide fit, bushing, trigger, barrel? I know this will depend on how it was built to begin with, but what are some reasonable expectations?

Before needing attention is a very broad statement, however. . .  If the barrel and bushing were properly fit in the begining I would say you could expect well over 20K rounds before you have to worry about anything and probably much longer than that.  Trigger is another story, I've seen some that have lasted for well over 20 years, and others that have lasted about 200 rounds.  Quality of parts, and proper fitting will be the determining factor of that process.

Keep shooting until you notice a problem.  If it ain't broke don't fix it.

Otiso wrote:How many rounds will a well maintained wad gun shooting lead bullets go before something needs attention. What normally is the first thing that needs attention? Is it:... slide fit, bushing, trigger, barrel? I know this will depend on how it was built to begin with, but what are some reasonable expectations?

Many factors to consider here, but most of the guns on the line have Kart barrels for a reason.   They shoot lead well, and lead wont wear the barrel as much as jacketed ammunition.  
     Different things can break for no apparent reason, and poor quality or incorrectly case hardened parts can be an issue.  

I would certainly take my guns out every twenty thousands rounds or so and sand bag them at fifty yards to see if they are maintaining acceptable accuracy.  

Any other problem will probably be of an acute nature.    Something breaks that keeps the gun from functioning correctly.  You will know it immediately.