Two years ago I found two mags that I had loaded during the come-home from Desert Storm. I was tasked with bringing automatic weapons from our demobilization post, back to our home armory. I loaded two magazines with ball 5.56 ammo and made the trip without incident. After we'd secured the weapons in the armory, I realized that I still had the magazines, so I dropped them in my ruck sack. threw the sack in the pickup truck and came home. This was in July, 1991.
I got busy after the war, and forgot about those magazines. I found them in June, 2011 and took them to the range for our qualifications with the Sheriff's office. I shot that ammo for qualification, and took the new ammo that I was issued as replacement stocks.
I know for a fact that those two magazines stayed loaded from July 1991 to June 2011. They fired just fine, no problems no glitches. That's one month short of two decades. I personally don't think it hurts anything to leave a magazine loaded.