I usually turn it off after the 2nd lap on any track.
I only ever turn it on when it gets dark at a track with poor/no lighting (eg. Mt Aso) & the headlights on the car I`m driving (which is most of them) are total crap, meaning you can`t actually see the track & have to guess where it goes by looking at the sillouettes of the trees.
It also makes some of the Mt Aso variations at night a hell of alot easier to drive quickly, as some of the bends have arrow boards, but the tighter, near 90* corners that turn the opposite way, mere meters after the signed bends have no markings or even barriers in some cases, so it`s impossible to even know where the track goes in the dark without the line.
If they made the headlights main beam actually light up the road ahead, instead of lighting up the scenery about 1 meter off the ground, this would help negate the need to use the line, as you could see far enough ahead to know exactly where the track goes.
[/rant]