here is question maybe that can give some insight too
on average how many pic do you get a week?
on many of my camera's that again run 24/7/365 they get 1500-2000 pic's a week ALL yr round!
and also, in what temp ranges do your camera's run and weather like environments, are they getting wet often, dampness, or in more dry places?
as this might add into life span or not?? who know's!