Another one of those once-a-month questions for which there isn't any answer likely to suit everyone.
I've asked this same question in more than a dozen armorer's classes over the years.
I've asked it of factory technicians.
I've asked it of a couple of spring vendors, one of whom is a major manufacturer of springs.
I've asked it of many other LE firearms instructors and armorers.
It does to seem to vary a bit from single column to staggered types, and somwhat among different designs, models and calibers.
Nothing really definitive has ever really been forthcoming.
They'll last until they become too weakened to provide for consistent desirable feeding and functioning.
Your experiences may vary.
I've seen some exhibit obvious signs of becoming too weakened for optimal consistent functioning within a rather short time, and then I've seen them last a surprisingly long time ... under similar conditions of having been left fully loaded ... and everything in between.
I've listened to other armorers discuss similar variable experiences.
I've heard it recommended by firearms manufacturers & LE armorer instructors that they should be changed whenever they start to feel like they're losing tension, or after a recommended service life (tracked by either rounds fired and/or time left fully loaded).
Not all springs are made of the same materials, or in the same manner, or intended to function under the same conditions or in the same manner.
Some springs are expensive, and some less expensive.
Specifications can vary, depending on the desires of the customer, even when the springs are made for the same purpose.
If a vendor ships a container of 10,000 springs, there may be some variation if springs are selected at random and tested to see how well they may meet the requested specifications.
Experts can disagree with each other.
I replace my magazine springs on a preventive maintenance basis nowadays, which can admittedly vary from one platform (including design, size, etc.) and caliber to another.
Predictable is preventable, to some extent, from a risk management perspective.
Folks will generally do as they wish, for whatever reasons, real or imagined.