my KA M4 metal mags are going on 5 years now I guess, 3 full seasons of HEAVY use. Only lubed them once, never misfed, still working great.
I had a TM G3SG1, I think 5 TM mags were 6 years old, and the new CA mags were 2 years old when I finally sold it. To my knowledge those TM mags are now 10 years old and still going strong. infact the gun is now easily 10yrs old and still working lol