That's kind of a weird question. Are you trying to figure out how long you should wait until you change o-rings? O-rings should be changed if you see an issue otherwise you don't need to change them. Pretty much goes for any marker.
yeah, thats a pretty obscure question. 14 cases is nothing. i would "guess" that hundreds and hundreds of cases would constitute "high mileage". In no way is that based on any kind of fact or good information for that matter, it is a simple guess. heck im sure pro guns get hundreds of cases put on them in a season, and then they get shot for many years after by whoever buys them from the pro. trust me ive never seen a gun that is to "old" to fix. dont get that confused with too busted to fix.....hope that uber uneducated or fact based guess helps.