But it depends on a lot of things.
My mom's S/O has a Tec-9 thats magazines have been loaded for about 10 years now. I unloaded them last year to prove a point, and he has to replace them all. He choose instead to reload them, and pretend it's not true. You can shake the bullets out of them.
Same dude, about 8 years back.. His 1911's mags had been left loaded for two months, and while they still would advance the bullets.. after 4 shots, it jammed. Feed strength on the springs had atrophied. Cheapo mags.
My 1911.. Springfield mags, left loaded by me being forgetful for three months. No issues at all.
Now, I do keep a mag loaded. Sits in the case with the gun. Locked. If I got time to get the gun, I got time to load and rack the slide. I unload and clean the mags every month. It may be paranoia, but I dont want it to ever go "Click" instead of "Boom".
The two Taurus mags I have for my PT-92 have held up like a champ when I've accidently left them loaded for a prolonged time. But I do have a handful of other assorted magazines that have crapped out on me due to compression. Rest in peace, original Ruger 10/22 mag. 
I can only speak from personal experience, but I've shot rounds through AR, AK, and HK91 mags that have been loaded for at least 6 months, if not over a year, without any issues.
For what it's worth, I leave the two mags for my carry piece, a Glock 29, loaded at all times. Haven't had any problem with them, either - though they get cycled a lot more often than my rifle mags. Arttemis fucked around with this message on 02-07-2008 at 09:24 PM.
As far as the theory on actually using/cycling the magazines causing more wear; The one swat dude I talked to at the gun shop I worked for actually kept a log on how many times his 1911's magazines had been used and how many rounds had gone through them. He always replaced them after X number of uses weather they seemed to have gotten *memory* or not. But I'm sure even high-end Wilson Combat 1911's magazines are quite a bit different than "modern styles" of magazines. I'd just say, Keep an eye out and give everything the attention deserves. If you take care of it, it will take care of you.
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Check out the big brain on Maradon!!
Revolvers also have springs in them, wtf.
Rested state until the the hammer or trigger is engaged. Sone have leaf prings that are always under tension, but it's a leaf spring. It's what they do.
But what Gork said is a whole bag of truth. There's no real excuse for leaving a mag loaded for so long. Company specs may say it's fine, but what if you got the dud that makes up the "exception".
