"Rotating" mags, meaning unloading one and loading up another, only puts more compression/decompression cycles on the mag, and THAT's what wears out a properly-designed spring. That and *overloading* the mag, which compresses the spring beyond it's design limit. That's why 10-rounders have such problems; because the mags are purposely designed to not leave sufficient room in the mag for the spring when loaded to capacity, thereby overcompressing the spring.
USGI 30-round mags are spec'ed for 6000 rounds, or 200 load/unload cycles, though they commonly last ten times that long.
Load your mag to full rated capacity with tested M193-spec, V-Max, or Power Point ammo and leave it alone. It will be ready when you need it.
And, if possible, pick up a factory (big $) or PMI/Precision Metal 20 or 30-rounder, and use THAT for home defense.
-Troy