Mine still look pretty new after 5 years, and up until last month it had never been parked in a garage. Omac suggested waxing them so I started doing that in June. Wax protects from UV which is what degrades the plastic.
I should add that newer plastics are better than they used to be, so even untreated lenses will go longer without fogging/yellowing
2013 Ram 1500 Lone Star Quad Cab 4X4. Bright White (PW7), 345ci Hemi V8, 8HP70, 3.21 gears (LSD rear). Mods list
2015 Durango SXT Southwest Heat RWD. White Knuckle (PW7), 220ci Pentastar V6, 845RFE, 3.45 gears (open). Huskyliners, Catch can. Thread