Mine generally last a two years or so, before they are scratched and fogged to the point that my vision is affected. The big culprit is dust - often metal dust, but also hardwood dust - in the shop. Even if you religiously use a non-scratching, micro-fiber wipe (which is hard), just dragging the stuff across the lenses damages them. Another culprit is glue, particularly CA (super) glue but also other types. You may not think it would end up on your glasses, but if you're using much of it, eventually microdroplets end up on your lenses. This does immediate, permanent damage to polycarbonate lenses.

My desk glasses, optimized for computer screen distance and never actually leaving my office, on the other hand - easily last a decade with no discernable damage. Because - no significant dust, no organic vapors, no glue, and always wiped with a clean micro-fiber wipe.