I had Xpel PPF put on the front of my truck and they also put it on the headlights, after 4 years they still look good. That said I have since heard a few people having issues getting the film off headlights when it does need replaced but it seems to be an issue with certain cars and trucks like the F150.
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