Sorry I'm late to the party, but in my experience, I get a year or less before a car that's outside starts to get yellowing again. I think Opti-Lens probably is best to put on headlights that haven't lost the factory coating, just like you wouldn't put Opti-Coat on a vehicle where the clear has failed already.
Just thinking out loud, Opti-Lens at a micron or two is much much thinner than a factory headlight coating, and I just don't think it can stop enough UV...and think about it...the factory coating may only last 5 years anyway (so what do we expect from a coating that's 10% or less of the factory thickness?)
If I was doing this for a living and wanted to offer a warranty, I'd look for a product that's more like the factory coating.
I loved the idea of Opti-Lens, but after repeated tries on a couple of cars (including one time putting on 3 coats), I always get the same results. On the plus side it doesn't take much to polish out the yellowing after a year.