I believe U need to get rid of them once the hole made by the firing pin gets too deel. After that, the firing pin won't, or barely will. make contact w/ the snap cap - That's the entire point of them. So, waiting for the outer part/front part of the snap cap to break is not the thing to do.
Jeeze, at $3 each, is it really worth it for a recreational shooter to spend that kind of money to practice? I know it's a personal decision; I'm not sure I want to spend that kind of money when it could go into ammo instead. I'm guessing I might be lucky to get 100 or so trigger presses per cap after seeing the condition after 30 presses.