There's 2 primary schools of thoughts on this. Some will swap their mags religiously, whereas, others will leave the same mag fully loaded in their HD gun for a year or more.
I personally down-load the mag by two for the nightstand gun and leave it in there for a year at a time.
The primary cause of magazine spring fatigue is the compression and decompression cycle. HOWEVER, leaving a loaded mag in a fully compressed state for months/years at a time DOES weaken the spring. If anyone doesn't believe me, load a mag and leave it the fully loaded state for a couple of months, then compare it to one that you'd fully loaded last week. There is a measurable difference in pressure. Is that pressure enough to make your pistol malfunction?
"You haven't lived until you've died." - various