Being one who used to design springs... spring design is as much art as science. There's a lot of variables. Some springs can be designed for "infinite" life, sometimes its physically not possible. It depends mainly on the geometry of the spring, how it's cycled, and what it's made of.
Simply leaving a spring compressed can damage it, but that's rare. The vast majority of spring damage occurs from over-compression. That's why you find bump stops in some magazines that only let the follower go down so far. If you remove the spring and mash it flat... you may have ruined it, though it might not appear any different. That said, leaving a magazine from any sort of modern firearm fully loaded for a very long time will not damage the spring as long as it's protected from corrosion.