I would have to say that it sounds very reasonable that the light and heat from the headlight or tail light might start to make the tint look more clear over time. I think this would be more of an issue with the headlights though. I would never paint my lenses because that seems like a irreversible process and if you ever want to sell the car that could hurt the sale. The only thing I can think of to maybe prolong the tinting of lenses would be using LED bulbs. I know this won't work for headlights but for the rest it could help. I really like that darker look though, I might have to see about doing that.