I've got mags filled with defensive ammo that haven't been cycled in about 7-8 years. Not all of 'em, but guns that have fallen out of my carry rotation (I carry a CZ P-01 Omega now and there are XD's and 1911's in the safe that used to be my carry guns) that still have mags loaded with defensive ammo in the safe.
I keep separate my mags I use for training vs carry. I don't want to worry about the carry reliability of a mag I just dumped on the ground that may have landed on the feed lips in a hurried change during a match or training class. I've also accidentally shot up my carry ammo a few times at matches. That gets expensive quick - carry ammo costs about as much as the mag it's in.
I don't worry about spring longevity in any of my modern magazines. My first XD is probably 15 years old at this point, I have its original magazines and springs, and that pistol has somewhere around 20k rounds through it. The original mags held 15rds and Springfield changed the bodies and followers slightly in later designs to fit 16, so I used the 15's exclusively for practice. Those two original mags have the majority of those 20k rounds through them. The only malfunctions I've ever had with that gun are after a 1000rd no cleaning torture test I did to it, and the failure was from the loaded chamber indicator on top of the slide getting fouled and blocking a round from feeding properly. Not mag related.
Guns that are in active carry rotation get the carry ammo cycled out about every 18-24 months, and the carry mags disassembled and cleaned and then function checked (usually the last mag of a stage at a match, to avoid dropping it or throwing it down). I worry more about pocket lint, gun oil and powder residue combining to create a problem than about the springs fatiguing.