glock makes some of, if not the best mag springs. i kept all mine fully loaded for over 2 years, including a 33 rounder, and they never jammed. (alternately took them to the range, then fully loaded again when brought home). changed them recently just to be sure. they're cheap as dirt, so why not? got them at glockparts.com
it's actually compression/decompression that wears springs out the fastest, not keeping them fully loaded. there's been 1911 mags from WWII found fully loaded, and they functioned without issue.