ck: I'm a bad boy in this regard and even moreso since the Original Ken responded as to how much of a good boy he is in this regard.
I have used magazines for 1911s tht have been loaded and left in desk drawers, glove boxes, tool boxes or fishing kits, for years. I 'try' to take all my different magazines to the range about once every 6 months when I go out to blast off ammo, kick cans or re-establish my 'skills' - and shoot the ammo I have had loaded into them for that time. I have never had a 45 magazine go south on me with that practice. The same applies to the magazines in my Tokarev, Berettas or Walthers. None have failed on me. But, I also make certain each one of them works properly before I start carrying the gun. I have seen and shot magazines full of ammunition that have been laying around for years. About 15 years ago I purchased a bunch of loose 45 ammo from a friend who said his Dad had picked it up years before - I thought it was WWII steel cased stuff but it turned out to be WWI cupro-nickled bulleted ammo. The box also contained two magazines that were fully loaded. They functioned perfectly - can't say the same for the ammo, but the mags worked just fine.
I wouldn't worry too much about leaving a mag full of ammo around for a while - just make certain it continues to work for you before you need it. Hope this helps. Mikey.