Keep them in the mag as long as you want. If you carry an extra mag you need to clean both every so often due to lint and dirt getting in them. I have 45 mags and AR mags loaded with ammo and they have been in there for over 20 years. I have beretta 92f mags that have been loaded for over 15 years. They will work just as fine today as the day I loaded them.
Springs are designed for compression and will return to their original length when the pressure is removed. They are not designed to be put through tension, so if you stretch a spring you will notice that they will not return to their original length.