Cycling glasses life span
Sunglasses have two enemies: dirt and UV rays. 

The polycarbonate itself is fairly tough and can last over ten years. It’s likely to  get scratched up before the UV starts to cause it to yellow or ‘craze.’ The coatings are more delicate. The anti reflective coatings are easily scratched especially if you use your sweat and dirt soaked jersey to 'clean' them.

The photochromic layer itself is activated by UV light but is also damaged by it. Generally, one can expect photochromic lenses to last 1-3 years depending on use. Keeping the sunglasses in a case (and away from UV rays) when not using them helps protect them.