We were told in armorer class back in the late 90's that S&W kept a lot of magazines fully loaded in their vaults for long term testing. They removed and tested them periodically. We were told that they'd gone out past 10 years and found normal function in some of their 59XX magazines.
The recommended method we were shown for testing magazine springs back then consisted of taking a fully loaded magazine, holding it upright (mag lips facing upward), with the nose of the top round facing forward (away from the armorer) ... and then briskly snapping/shaking the magazine forward a few times, as if trying to shake water off the top of the mag, to see if the top round could be dislodged by this motion. If it could, time for a new mag spring.
However, in the early 2000's they changed their armorer recommendation for duty guns to replacing recoil & mag springs either every 5 years or every 5,000 rounds fired, whichever occurred first (with the presumption that duty mags were being left fully loaded all the time for that 5 year period).
The instructor acknowledged that this was probably a conservative recommendation, intended to err on the side of safety when it came to duty pistols that needed to demonstrate optimal reliability (and probably under adverse and difficult conditions).
I've run some 9mm range mags for many years, constantly loading/unloading them (which creates wear on them). Most of them ran fine. Some didn't, failing to feed the last couple or few rounds.
I've also had duty mags (constantly loaded, but not used for excessive range work) start to fail to lock the slide back on an empty mag, as well as start to fail to feed the last round or two. Not a good thing. Better to have it happen on the range, instead of off the range, though, when good feeding might be critical, right?
So ... over the years I've decided that it's better to err on the conservative side of things, and I prefer to replace my mag (and recoil) springs on my 3rd gen guns at least as soon as recommended in the armorer classes, if not sooner.
Besides, we started issuing +P+ and then +P loads instead of the standard pressure 147gr loads as time passed. Those can produce increased slide velocities, which can not only accelerate wear on the recoil spring, but the resulting increased slide velocities are "less tolerant" of weakening mag springs when it comes to the springs having to keep the rounds up against the mag lips quickly enough for feeding to occur within the intended "timing".
In the greater scheme of things, mag & recoil springs pretty inexpensive, especially if you buy factory springs.
Then again, some folks like to run their tires so long that the belts are showing.

Probably doesn't allow for the same degree of traction in stopping and cornering when they're that worn, do they?
Just my thoughts.