I've left magazines laoded for many years, a few as long as 25 or so. They all seem to work fine.
I'm not trying to argue and I leave a bunch of my mags loaded. But I have seen magazines fail. For instance, I have a WWII M1 Carbine. I put the GI green sleeve on the buttstock that holds two magazines. Put two loaded GI 15 round magazines in that sleeve. (These mags looked brand new.) Left it that way for.... I don't know, 3-4 years. If I wanted to shoot the rifle, I just left those two mags there. I decided to fire them one day and took the rifle out in the pasture. Both mags failed. There would fire maybe half the mag and then just stop without raising the next round. It's like the mag spring wasn't long enough any more to push the carrier all the way to the top.
I threw them both away. And I don't keep my M1 Carbine mags loaded anymore. Maybe it is a design thing with that mag... I decided to "take the hint."