John Dean Cooper wrote on the subject from both personal experience and observation. To the best of my knowledge the longest storage Jeff Cooper ever wrote about was about this example:
That was a 1921 Commercial Colt that laid in an attic in Condition One from the day its owner died in 1929 until my step-father inherited it at his aunt's death in 1991. He died in his sleep, and his wife found the gun in the nightstand. She wrapped it in a diaper...put in a hat box...and stored it in the attic. Her surviving sister gave it to my step-father the day of the funeral, saying that he was the only family member who would have an interest in it, since Aunt Emma had outlived all her sons, and her sister was a spinster. The gun has been willed to me.
John T..... who did at least one experiment on his own:
.... I left 18 magazines equipped with Wolff springs loaded for a year...and then pressed the magazines into service for range use. Five years later, they're still there.
That was several years ago and I haven't heard to the contrary.
There were any number of examples left loaded during deployments of varying length - lots of 7-10 year examples, quite a few duration of the war examples and a few from WWI to WWII examples.
A little googling will show results like: American Handgunner, May-June, 2003 by John S. Layman:
Magazine spring madness: 'creep' to your 'elastic limit' to un-earth the urban legend of 'spring-set' which mentions examples up to 50 years.
Colonel Cooper was AFAIK something of a pedant on clips and magazines though. Any WWII clips he tested were I expect for the Garand and lacked any separate springs.
the use of "clip" in place of magazine and such-like barbarisms are annoying
Cooper
I suspect that unlike the coil springs in magazines the flat brassy tensioners in some stripper clips might well lose tension over time as steel flows slowly if at all at room temperature and certainly copper alloys will flow more but I don't know how much more. I have stripper clips that have been loaded for many years - with empty cases - and still hold the cases in normal handling.
