I don't really follow any set pattern. I have had the same ammo in my 220 for over a year. Ammo doesn't seem to really go bad from my experience just because its in a gun. Now for piece of mind if something happened to compromise the integrity of the ammo I may shoot it up.
I can see rotating magazines to reduce the wear on the springs. I think I may start doing that since I'm down to two semi auto pistols.