For a significant number of years I worked part-time doing sales and repairs in a moderate sized gun shop. Besides new guns, we dealt a lot in older, used, classic guns. It was very common for people to bring in old guns and accessories for us to buy or sell on consignment, frequently after the death of a loved one. We would go through this stuff and it was surprising common to find many of the guns or magazines loaded, especially hand gun magazines. I can attest to finding many of the springs in pistol mags ruined from being left loaded apparently for years. I've seen the springs on a few mags so worn out from being left fully loaded for years that the follower wouldn't even go up to the top of the mag when emptied. This was the exception though. The normal mag ruined from being left loaded would function in the pistol OK for about the first 2/3rds of its capacity and then begin to exhibit failures to feed for the last few rounds.
Over the years I've seen a lot mags with bad springs. I'm sure that all mags and springs aren't created equal, some are certainly made better than others. However, of those mag I've seen with bad springs, they weren't worn out from being cycled up and down like automotive springs by constantly loading and unloading them. They are worn out by taking a "set" from constantly being under a full load.