Obviously, it would be best for the spring to unload the magazine immediately following each use.
Do you take the load off your car's suspension and valve springs each time you put it away for the night? A spring can sustain a static load within its elastic limit essentially forever. Steel structures all around the world bear witness to that. Corrosion, large numbers of load cycles, or overloading them are the sorts of factors that will put paid to a spring, not simply leaving it loaded.
You might find this interesting:
https://www.24hourcampfire.com/ubbthreads/ubbthreads.php/topics/12760659/1
