20 years on my first Leopold (2.5x7 with the friction as they all were originally) after being dropped it went bad. Sent it in and got a brand new one back for free, I'm going on 18 years with this one. My dad has had three over his 50 years of hunting and not one problem among them.
I'm guessing three things are going on. 1. Wind/weather conditions 2. Ammo changes (I've seen impact changes of 5-6" even with the exact same ammo, but from a different lot) 3. some problem with the rings.