Can you leave your magazines loaded with cartridges, for long periods of time, without ill effects?  Yes, you can.

A magazine wouldn't be a very useful tool if it didn't do it's job well.  The job of a magazine is to stand ready with a supply of ammunition, waiting for the moment the gun calls for it.

In the military, as well as while concealed carrying as a civilian, I have personally owned magazines that were constantly loaded for years.  The only time they went unloaded was when I used them, and then they were promptly reloaded minutes later.  I have yet to have a magazine that developed what I would classify as a spring related problem.

My solitary experience doesn't exactly qualify as statistically significant, and there are lots of people out there that will tell stories of bad magazine performance.

Modern metallurgy and engineering produces great results.  Springs are stronger and longer lasting than ever.  Firearms engineers understand the demands placed on the guns they design, especially in the defensive shooting industry, and they take great care to ensure their creations will meet or exceed those requirements.  With this said, seriously consider purchasing only factory magazines for your firearm, instead of "budget" magazines from other 3rd parties, if your life may depend on the reliability your ammunition feeding source.

So load those magazines up, and keep them loaded and ready.  Take them shooting with you, use them, reload them, and rotate them.  Get to know your equipment, and have confidence in it.