How long would you leave a magazine fully loaded for before unloading it to let it relax? Also has anyone here have any experience with +2 or +4 magazine conversion kits on the market?
Depending on who you ask it is either compressions/decompression cycles or time at maximum compression that wears out springs. I have a 870 shotgun with a magazine spring that got weak from being fully loaded for several years so I am not buying the current theory that cycles is all that matters.
The magazines for my 22 compact are never loaded for more than a few minutes since it is strictly a range gun. But in general I would not worry about it and leave them loaded for as long as you want. They will get weaker but should still work fine. I heard of some older designs like 1911 magazine springs getting so weak they cause problems when left fully loaded but modern ones do not seem to ever have a problem.
Unloading them to "relax" them will not do anything to reverse damage.