Every so often someone asks whether leaving a magazine loaded for long periods of time might weaken the magazine spring to a dangerous degree.
The consensus answer seems to be as follows:
Yes, keeping a magazine loaded for long periods may result in the spring weakening to the point that cartridges will not feed properly. This does not happen with very often, and is less likely in magazines which use higher quality springs. Incidentally, this is one more reason to prefer factory magazines to aftermarket ones.
The solution is to obtain an extra magazine spring (generally available wherever you bought the gun, or from the larger mail-order houses like Brownell's). Don't ever put this extra spring in a magazine - instead keep it as a comparison sample. Each magazine which is kept loaded should be taken apart every few months, and the spring should be compared with the extra. If the kept-loaded spring is noticeably shorter or weaker, replace it. Also, practice with the same magazines which you keep loaded, so that you can notice and deal with any problems in a non-threatening atmosphere.
There are aftermarket magazine springs which are claimed to be better (stronger, less likely to weaken or take a set). Wolff is the best-known brand. I have no personal experience to confirm or deny their supposed superiority, but they are not overly expensive and many people seem to like them. If you put aftermarket springs in your magazines, you really should re-run your self-defense ammo acceptance tests - remember that any change in the cartridge feeding system can effect reliability.