Keeping ammo in magazines: Bad idea? How long is too long?

[punkndisorderly]

There are probably a lot more variables than we are taking into account:
-there are lots of different designs and materials used in magazines, both between different firearms and even within each model (how many different AR magazines are there?).
-differences in heat treatment between batches
-differences in manufacturing methods and materials over time

It could be that some magazines are fine to leave loaded and others aren't.