Magazine springs get worn out from the compression and release of the spring during loading and firing as well as from being compressed for long periods of time when magazines remain loaded. There is no simple answer. Personally, I rotate my carry mag spring out every two years. Another good suggestion is to keep a new set of springs. When you do your annual maintenance, take your used mag springs out and compare their length next to the new ones. If the used one show significant difference in length, replace.