Magazine springs are pretty inexpensive parts, all things considered.
When a spring vendor ships upwards of 10,000 springs per case, do you really think that each and every spring is going to be identical if pulled at random and checked on the proper equipment? Maybe there might be some variance within an expected tolerance range?
I've been through a number of different armorer classes over the years, including some Glock classes, and the same question always comes up. Not unexpected. Everyone has to learn something for the first time at one time or another, right?
The instructors have offered different answers at different times, sometimes changing over time as I've attended subsequent recert classes.
Some of them have gone to conservative recommendations for replacement schedules, erring on the side of safety when it comes to agency weapons which are dedicated to service use.
Some recommend frequently checking for proper tension with inspections, bench checks and during live-fire.
The related experiences of other armorers has varied quite a bit, too. Makes for some interesting and sometimes lively discussions in classes, especially when experiences differ among armorers.
I've seen some springs run reliably after being left fully compressed for many years. Others exhibited signs of weakening and caused feeding stoppages after only 2-3 years of being left fully loaded. Roll the dice, if you want ...
I remember in my first Glock class the instructor waffled about a specific number of years before mag & recoil springs had to be changed. As students sensed his discomfort in providing a definitive answer, they called out different lengths of time. The instructor finally stopped the discussion by stating that 8 years (the last number called out by an attendee) was too long to leave a mag loaded and carried.
Other Glock classes have yielded other responses, recommendations and suggestions from instructors since then.
I've seen a list sent out by Glock which recommended magazine spring replacement every 2000, 2500 - 4000 or 5000 rounds, depending on which .40 model was listed (Glock Wearable Parts Replacement Schedule dated sometime in 2008), but also recommending that for best results, to replace the magazine springs at least every other time the recoil spring assembly is replaced (or whenever inspection indicates it would be prudent to do so, naturally).
The newest manual pretty much just identifies the mag springs as one of the several springs that may require attention and replacement at some point (leaving it up to the individual armorer to inspect particular guns and make the proper determination).
Me?
I've seen Glock mag springs work fine after having been left loaded in pistols which saw very little actual shooting use (meaning infrequent qual and little or no practice use), sometimes for up to several years ... and then I've seen stoppages resulting from obviously weakened springs which according to the owners had only been in the mags for a couple of years, or no more than 4 years (one such individual said he only fired the gun for quals, which was 4 times per year).
I've seen similar things happen with mag springs used in other pistols commonly seen in both LE/Gov and CCW use.
If I had a dollar for every time a qual range was delayed while someone had to resolve repeated stoppages caused by obviously weakened mag springs ...
and the owner told me the mags had been left loaded for years at a time, but were seldom (or never) used for actual shooting/practice that wasn't required for either off-duty or CCW licensing ... I'd have been collected enough money over the years to buy more firearms.
I've talked to various company reps and other folks who work for spring manufactures, as well as technicians, managers, reps and even an occasional engineer who works for one or another of the firearm companies. Their opinions and experience can vary, too.
The important thing ...
to me ... is to periodically inspect and monitor the condition and functioning, during both bench checks and live-fire.
This means I pay attention to the way the springs feel and function each and every time I load and use a magazine, as well as when I disassemble and inspect them for cleaning and/or periodic armorer inspections. An observed (or even suspected) problem with a spring results in a new spring.
Preferably, I replace the springs in "working sets" at the same time (issued duty mags), based upon a service/usage interval, but a repair replacement for a specific spring found to be defective, damaged or one which simply failed for an unknown reason sooner than others will get replaced when identified.
Suit yourself. Everyone else does.
I prefer not to take unnecessary chances with the pistols used as dedicated defensive weapons by either myself, or others for whom I'm responsible to help maintain & support their pistols.
Just my opinions, albeit those which have been influenced by what I've learned and heard during a fair number of years while attending different armorer classes, as well as while helping support a fair number of pistols, rifles and shotguns (all of which use mag springs), and after having worked as a firearms instructor for more than 20 years.
The first few times you see someone's duty pistol experience feeding stoppages due to weakened mag springs, on a qual/training range, you suddenly become very grateful it happened on a range and not somewhere else. The discussion topic becomes a bit less hypothetical and more serious when you realize you're talking about dedicated defensive weapons actually being carried into harm's way every hour ... and not just a pistol being used for leisurely range enjoyment or some competition venue, when all that's being risked is the loss of points.