Parts

i think with everything it would be a case to case sort of think but with that said moving parts are going to wear the most 

but what to buy would be a guessing game at best 

and this would be in every type of pistol or rifle, it is a crap shoot

Not mentioning a specific model really limits the information you'll get back. It's always good to have extra o-rings for ANY gun but different makes/models have their own idiosyncrasies. The more information you put in your post, the more RELEVANT information you'll get back from the people with relevant experience. You're kind of tying one hand behind YOUR back and OURS! Like saying "when do I need to have my car worked on?" with no other info!

I would have to say the valve stems go bad about as fast as the orings in my experience.