Typically, I'm given to understand through reading on the topic (including a similar thread here a few months back), that prolonged loaded condition of mags should not have an affect on their functionality.
Besides, given that I fire half of my guns on month one and the other half the next month on a rotating basis, those mags get "releaved" of burden every other month or so.
Having said that, I typically have four mags for each of my autos. One in the gun, ready to rock and roll, two loaded for back ups and one given a rest until the next time I rotate the mags (i.e. fire them out). Having that "spare" also allows you time to find a replacement if one of them goes bad. (I've only had one mag ever go bad on me. Couldn't fix it no matter how hard I tried.) So, no worries and with care, mags last much longer than the barrel lining of the weapon will.
GG