This subject is a pretty simple one, but the internet and gun forums always seem to stir up debate about it.
I've listened to armorer instructors, other armorers & firearms instructors, repair technicians/gunsmiths, engineers and spring companies discuss this subject.
Springs can become weaker over time, whether from being left compressed or from constant use.
Some spring materials seem less susceptible to some wear than others, but it also depends on the application.
I've been to over 20 armorer classes, and have seen a lot of different magazine springs used in assorted weapons (box & tube). In armorer classes the recommendations can vary according to the manufacturer, but they all recommend periodic inspections by armorers for proper spring tension & function, in addition to owner/user checks each time the weapon is used, cleaned, etc.
Some manufacturers offer replacement recommendations based upon service usage (round count and/or time in service).
I've seen different design/style magazines offer differing spring service life characteristics.
I've seen some last longer than expected ... and some not last as long as normally expected.
I've seen caliber and ammunition power levels have an effect on things, especially when it comes to recoil springs, but also when it comes to feeding "timing" and mag spring tension & service life.
Bottom line, I check for spring tension using recommended bench checks discussed in the armorer classes, as well as by observing performance and functioning on the range.
I replace any mag spring either as it may seem prudent (based upon checks), or based upon a bench check (failing to hold the slide locked back on an EMPTY magazine when briskly running the slide), or at least as recommended by either the gun or spring manufacturer. Mostly sooner, as I prefer to err on the conservative side of things in the way of critical functioning matters.
I prefer to replace my own magazine (and recoil) springs before
I start experiencing stoppages and malfunctions on the range caused by weakening springs. I definitely prefer to replace them before
I might experience a problem off
the range, in actual usage.
The magazines I leave loaded for extended periods get new springs periodically, usually as a "group", and I mark the mags when the springs are changed (marker ink dating on tubes, or silver/gold marker ink on floorplates).
I can't predict a premature failure due to breakage, damage or an unknown defect that causes early failure ... but I can try to make sure I stay ahead of the normal curve when it comes to periodic maintenance.
FWIW ... I've seen a significant number of stoppages & malfunctions caused by weakened mag springs occur during courses-of-fire over the years. These were in both issued and personally-owned guns being carried by both LE and non-LE (think CCW carriers).
The users & owners were all pretty consistently surprised when it happened to them. When questioned about when they'd last replaced their mag springs, the typical answer was a blank stare. Most said they were using the original springs, and that they'd been left loaded for anywhere from a few years to many years.