They need changing when you start to get failures to feed, which is uncommon. You can create a situation where you may need more powerful magazine springs by using an overly strong recoil spring.
Jeff Cooper once did an experiment where he fully loaded a bunch of GI magazines and put them away. As I recall the story, he would pull out one magazine a year and shoot it, trying to determine how long they could take full compression. After umpteen years, they were still working fine, so he did not find out the answer to the question.
It may be that cycles of alternating compression and extension have more effect on spring life than extended compression.
I compare the current spring to a brand new one when I clean mags. Since it's already disassembled it's easy. BTW - the springs have to be the same brand.
If it's a lot shorter, I may change it. Have replaced a few - almost always a mag I've "inherited" from someone or came in a "deal". If a new spring and Metalform follower won't make it feed, it gets to meet my ball peen hammer
[This message has been edited by JSP (edited 10-13-2000).]