Too many variables to nail it down to a real number.
The weight of the charge and cleaning being the 2 biggest issues, and what kind of accuracy the shooter wants.
Gas cutting happens, it's a real thing. That powder going off want's to leave the barrel and it doesn't care what direction it goes. So each shot pushes hot gas back up against that little hole in the nipple.
Sooner or later that hot gas starts making that little hole bigger.
When it get's bad, you will get hammer blow back, the escaping gases from the nipple will push the hammer back even as far as the half-cock position. That gas escaping the nipple changes how much gas is pushing the ball and groups change.
Now I enjoy target accuracy, I want to hit the X, I want to cut that string.
I use sporting grade powders and pretty hefty charges,, long story short, I notice my groups opening up around 400-500 shot's, changing out to a new nipple changes that.
The same nipple I consider worn, will last someone that want's "minute of Deer" accuracy another 500 shots or more.
I consider $5 for a new nipple a few times a year pretty darn cheap.