According to my University Physics book I have lying around, metal springs can be "worn" by stretching them past a certain physical limit. For example, a slinky will coil up nice and small until you stretch it too much, and then there's a coil or two which won't go back just right.
As long as you don't exceed those limits (such as removing the spring from the magazine and pulling it apart really far), the spring shouldn't experience any significant wear. Now, significant use of long periods of time (decades) may cause the spring to weaken, but that's not usually an issue.
Think of the small springs in your car's engine (and the large ones on your car's suspension). They are usually found under some degree of compression, and are "cycled" millions of times in a few hours of driving. How often does one change the valve springs in one's engine, or even think about changing them? Probably not very often, unless something has gone horribly wrong. Same kind of thing.
