The spring will last indefinitely under full compression. As others have stated, it's cycling between empty and loaded that fatigues the spring. And fatigue life of steel is measured in 10's of thousands of cycles to over 10's of millions of cycles. You ain't gonna hurt it.
The feed lips of the mag don't actually get an impact when the gun fires. The cartridge hits the bottom of the bolt, not the lips, as the bolt strips the round above it forward into the chamber.
The only problem is with plastic mags, kept loaded, but not in the gun. Plastic creeps over time, spreading the lips. That's why Magpul PMags have a plastic clip on the bottom that can be put on the top when the mag is loaded, to keep the top cartridge off the lips, like they would be in the gun with the bolt forward.
Aluminum does not creep, so those mags can be loaded all the time, in or out of the gun.
NRA, SAF, NRA-ILA, ACLDN, IDPA, PDN
"Give me liberty or give me death." - Patrick Henry
I'd rather be carried by 6 than caged by 12.
2016: It's pronounced twenty sixteen.