A quality mag can stay loaded for many years and remain functional. It's the act of compressing and relieving the spring that causes wear.
I found a couple of fully loaded Wilson 47D mags in a drawer that hadn't seen daylight in many years. Both mags functioned flawlessly when I emptied them at the range.
I've also heard of AR mags being loaded for over 20 years and firing all rounds with no problems.
I am scared of empty guns and keep mine loaded at all times. The family knows the guns are loaded and treats them with respect. Loaded guns cause few accidents; empty guns kill people every year. -Elmer Keith. 1961
In general agreement with what HV said. However, I had a mag from Marlin .22 carbine bought back when I was a kid that got to where spring was so weak it would not push last round to the top. But ... it may tho have well been from those thousands of rounds that I ran thru it over the decades rather than having left it loaded ...
I found an old Butler Creek 30 round .22 mag in the top of my closet the other day. I would estimate that this mag was loaded five years prior to the AW ban. I put it in my 10-22 that I have not shot since before the ban and it functioned absolutely perfect.
Jason
NRA Life Member
TSRA Life Member
"No man stands so tall as when he stoops to help a child."
"It depends." 
and it functioned absolutely perfect.