Generally, leaving a magazine loaded will not hurt the spring.
The thing that does wear the spring is constant loading, and unloading. The compression, and decompression is hard on the spring, and will weaken it over time.
Just leaving a spring decompressed, or compressed will not hurt it. You should be able to leave your magazines loaded indefinitely without a problem.
If you dont want to do that, then rotate your magazines. Load some up for a while, then unload them, and load up your others. Just don't do it too frequently, or you will wear your springs.
nWo FOR LIFE, Brother!
"The AK-47. It'll never jam, never break. Rugged, reliable, and so simple to operate, children could use it....and they do" Yuri Orlov. 'Lord Of War'