When I was shooting Trap, I was shooting 4-6 rounds per week. That is 100-150 shells per week. I would venture to guess when I was a duck hunter I shot a few boxes or so prior to duck hunting season, another couple boxes during hunting season and none the rest of the year.Obviously how many rounds you regularly put through it depends on how frequently you shoot and what you shoot. Trap and skeet are 25 targets per round, whereas many sporting clays courses are 100 targets per round.
That doesn't seem like much but it adds up quickly if you get addicted.
So I'll call it less than 150 shells per year.