Springs wear out from being cycled/used not from just sitting compressed (within their spec range). I'd say keep them loaded until you're ready to go shooting. That's what I've done for years without any issues. I do have range mags I use many times as well so I don't have to unload and reload my carry mags.
Steelheart is correct. It's not the compression but the cycling of the springs that were them out. Keeping your mags full puts less cycles on the spring then emptying for storage. I have read this from several people that know much more then I about metalurgy.
Think of the springs on your car. Its not the weight of the car that wears out springs but all the bumps in the road. Thats why shock absorbers are installed.They will absorb most of bumps and not the springs. The springs take the weight of the car and everything in the car and keep them off the frame.
Coils springs are much longer lasting then leaf springs. Just look at old Corvettes with a transverse leaf springs on the rear axle. The cars that have spent more time on the road will have the rear wheels canted outward. One thats hardly ever driven won't show this cant. Now matter how old the car.
A few years back I also kept some mags (Ruger 10rd mags for a P95) for over a year without any feed issues. And I know that the carry mags for my XD have been kept load for the better part of 3 years.
Now, that being said. I would still download a 30rd AR mag (and maybe a 20rd) by 1 to 2 rounds, no to attempt to preserve the magazine springs but to increase my feed reliability. But thats another topic for a different part of our board.
[/ Carlton Nether, Customer Service for Beretta USA, tells us keeping a pistol magazine loaded for an extended period doesn't cause magazine spring failure, however, failures to feed can result. He says, "The ammo will 'roll' in the magazine. If the mags are kept loaded and moved around a lot -- say on a cop's belt -- the rolling action can, over time, cause creases in the cases. These creases can cause malfunctions. Also the top bullet will roll against the magazine lips and creasing can occur there as well. Just check old ammo that's been bouncing around in a magazine for a long time.
We tell police officers if they keep loaded magazines, take a few seconds to "cycle" the ammo. Periodically unload the mag and reload it in a different sequence. This movement will allow the bullets to be in different parts of the magazine and help eliminate creasing.
Above is from the link Steelheart posted. This is why LE agency training units want the ammo cycled every couple of weeks in your duty firearm. This I knew already-- however, until now, I was under the impression that keeping the mag spring compressed will result in reliability issues. (Me with my thick head took the word of an old timer in the Academy for all these years about this subject) Upon my own research and talking to officials from AZ DPS and the Maricopa county Sheriff's office, I realize keeping a mag loaded for an extended period of time will not affect its performance. (Unless it is a duty weapon---see above in blue) To this end I retract any former statements that I made about the weakening of mag springs due to constant compression.
Thanks again guys! I can see how movement of the ammo in the loaded magazine can cause the springs to wear. I will just try not to bounce around my mags too much. I will likely just recycle rounds through the mags every few weeks to play it safe... although I think I will just go shooting more often to solve that little problem!