Not sure how long they can be loaded. I used some loaded mags for my 1911 last Tuesday. They worked fine and I reloaded them three times on the range and they are loaded now. But these mags have been sitting on the shelf, fully loaded for over a year.
I have some Glock mags that have been loaded since 2003 when I moved into this house. Have shot some of them lately but got about 15 more still loaded. Might be a few years before I shoot them since I stopped shooting any of my Glocks months ago.