First of all, I just registered and I'd like to say hello to everyone and Semper Fi to all my Marine brothers and sisters.
I have an AR15 mock M4 that I use for home defense. I usually keep my C-Mag loaded with 10 rounds of 55 gr surplus but that's rather large and cumbersome sometimes, even with a partial load.
I'd like to use my GI mags but I don't know how long I could keep them even partially loaded before the springs fatigue. And I'd rather not do some destructive testing and find out.

Any advice would be appreciated, thanks.

One member here found a 20rnd magazine from Vietnam loaded in his foot locker a few years back.  He took it to the range and shot it no problems.  

If you are concerned about spring fatigue there's two things to worry about.  Excessive loading and unloading of the magazine, and overloading of the spring.  Springs are relatively cheap, check [url]http://www.gunsprings.com[/url].  A regularly loaded spring in a static position should NOT experience much wear at all.  if it does....it's crap and should be replaced.  

Thanks, guys. I'm in Hoffman Estates, BeeKeeper. Nice to meet someone nearby. :D