I would say years and years. If it starts to fail to load the last round or maybe if your slide fails to lock back on your last round fired even that could be your followers are even a worn slide release. Anyway you can get them here. Magazine Parts-www.glockmeister.com
I've owned Glocks of one kind or another for more than 16 years and have never replaced any mag springs. Most pistol mag springs will last for many, many years regardless of what you read on the internet. A good deal of the recommendations for replacement and stories about them wearing out come from spring manufacturers. I've got a Colt Woodsman (made in 1947)in the safe that has been loaded and not fired for more than 30 years that worked without flaw last fall loading every round. I doubt the mag springs have ever been replaced.