Metal will maintian its elestic nature in theory forever. Esentially when the metal spring cools the metal crystals freeze in the springs relaxed state.
A spring will return to this state until you stress it so much that the crystals adopt a new shape. This is why you never want to "over stretch" a magazine spring.
As for leaving it compressed for a long time that should not be much of a problem. It would only be a problem if something called "creep" happens. This in essence is that the metal crystals "creep" and adopt the new orientation.
Now this is all theortical and may not reflect your actual experience. But from a Material science perspective a spring should be able to be compressed and relessed as much as you want as long as you do not stretch it or compress it past its level of elasticity. If you go too far you ruin the spring.
I read that some 30 round tommy magazines from WWII were found, still loaded with ammo. They were loaded and fired great.
There are many opinions on this one, but I'm with Gokyo on this one. Steel that is not stressed beyond it's elastic range (statically) suffers no permanent deformation (aside from creep, which should be minimal.)
Worn springs are likely due to fatigue, which is caused by repetitive (cyclic) loading even inside of the elastic range.
As long as you don't over stress the metal springs...........Springs existing in a static state should not effect their performance.
However, having said that ......... I take these added precautions
1. I never load a magazine to full capacity, if I plan to store it loaded for long periods. 1 or 2 rounds shy is good.
2. I always try and rotate my magazines every few months.......and allow them to remain in the relaxed state for a short period of time.
Springs will take a "set" after use ......... so if you notice that some older springs are "shorter" than a new one........this condition can be normal. Many spring manufacturers will take this "set" into account when designing the spring to fit a certain application. So NEVER , EVER try and stretch a spring back to its original length. Either replace it [ if it is showing signs of weakness ] or leave it alone.
How long will it last fully loaded? Depends on who you ask.
This is a tip specific to knowing if YOUR mag springs (not some internet guy's spring) are weak. Hold the gun vertical, pull the slide back, and release it slowly untill the slide stops. If the slide stops in full battery...smile. If the slide does not go into battery on its own 'I' would get a new spring. Stretching the mag out will bring back its tension... temporarily.
Now I would think that if you are going to 'underload' then doing two rounds in a double stack magazine would be best. Underloading one round would not relieve significant tension since the other round is still compressing the spring that same distance. (Edited - I add this sentence to note that this paragraph makes no sense at all, please disregard. First impulse + lack of thought = public humiliation)
To prevent your springs from going bad make sure they are protected with rust inhibitor lube and check/clean every few hundred rounds or so...those things will rust...but then again...I have not tested this myself.