If you start with lots of spares, it works for a pretty good amount of time.

Let's say your family has 5 full time members and is timely at doing dishes. 15 sets of flatware would probably be plenty, but sometimes you have extra guests, so start with 20-25.

If you lose or break items and get down to 15, and the set is no longer available, get rid of what you have and get a new set of 20-25.

Don't care also works fine.