I just switch out every month or two.
And I don't care WHAT people say.
Leaving a magazine loaded WILL weaken the spring... I am not saying it won't function, it would have to be left a HUGE number of years to actually fail in functioning.
Proof? NIW AR-15 and Mini-14 magazines. One half dad loaded, then left in a can, the other half, unloaded in the wrapper. Result=easily noticable differance. This is after 7 years loaded with very few runthroughs.
Likewise a Ruger P85 magazine.....Ruger factory and almost fails to feed, not quite yet. But the spring is so weak that it is the problem....Stayed loaded with all 15 rounds for over 8 years.
On a end note, I also often leave 1 round out of the magazine if they will be loaded over long period. ALso in speed, it also is easier to seat the magazine.