The springs mostly get weak from the repetitive loading and unloading of a spring causes something called elastic fatigue of the metal.... leaving compressed does not really hurt the spring unless LONG term it can weaken slightly; known as CREEP, in which a metal held at a constant strain will gradually relax and so reduce the stress. This will reduce the stored energy.
Industry standard is spring last for about 5 years of use...
As a side note.... mags are a Replaceable item and do malfunction for a number of reasons which is why you should prob have about 10 mags per gun for long term.
Also had 6 Sig mags loaded ALL the time for 30 yrs ( other than unloading once a year to swap out rounds with new ) , never had an issue and they still work fine 17 yrs later.