FWIW, I do not know how common the breakage is -- I've never seen it personally -- but two folks at Glock both recommended to me that I replace the magazine catch spring with some regularity. While breakage may be rare, if it snaps in two below the frame hole your pistol is essentially deadlined permanently. They've only seen them break when using the extended mag catch (which I don't run) due to the additional flexing, but I would hate for my test & carry gun to die.
My plan at present is to replace the recoil spring every 7,500 and then every 15,000 I'm going to replace:
- recoil spring
- firing pin block spring
- striker spring
- mag catch spring
- takedown lever spring
It's also been suggested to me that the slide stop lever should be changed every 15k or so. I've got to call LAV and see if he can send me a few spare levers.