I don't have the widest variety of firearms, neither the most rounds downrange, nor the longest record for having kept magazines loaded, but from my experience it doesn't usually matter about keeping magazines loaded.
I worry more about bullet setback or primer failure on my defensive rounds than my magazines going bad. Although, in reality, I've never had these problems either.
I had one old 10-round rotary magazine for my Ruger 10/22 which I have owned since 1990 and have numerous thousands of rounds through, that started failing due to a weak spring. The rounds would simply get stuck and no longer feed sometimes... we'd shake the mag and rounds would start falling out. I disassembled the thing and cleaned it out, which helped for a while, but it continued to fail. My other, similar mags had noticeably more spring tension. I replaced the spring with an aftermarket spring from TandemKross and it works fine now. I figure that spring was probably weak from the factory.
Other than that, I've never had a problem. Factory Ruger Mini-14 20-round magazine that I bought used in the early 1990's stays loaded and in rotation for my home defense rifle. Glock magazines I kept loaded for 15-20 years worked flawlessly with all old ammo, even after having been exposed to extremes of heat and cold being left in my car. My CZ carry mags end up staying loaded all the time unless I'm burning through some older defense ammo to test something or cycle it out to replace it with new ammo. I have separate range mags which don't stay loaded, but see a lot more use, and still haven't had any failures with them. I disassemble them and clean them with Eezox periodically. They have obvious wear on the steel and the polymer followers, and slightly compressed springs, but work fine. I have spare parts... springs, followers, base pads, and whole extra unused magazines, so I'm sure I can piece together a good set of working mags anytime for my P-07's.
