If you can see the area lit by their headlights, then you are probably close enough to dim your own.
You should dim your headlights within 500 feet of the car in front of you. The rule of thumb is, if you can see another car, dim your lights.
never
Keep your headlights on low beam.
500 feet if the vehicle is coming to you, 300 feet if you are following it.
Common courtesy would be at least 500 feet.
at least 2m
In most states it is 500 Feet. Common sense should prevail.
It perfectly depends on where you live or drive. You should dim your headlights right when you see another vehicle in front of you or right when you see the lights of another vehicle. It doesn't matter if you are seeing the tail lamp (both of you are traveling in the same direction), you should dim the lights or else it is going to make difficulties for the driver in front.
200 Feet
Headlights should be dimmed as soon as the driver is aware that another vehicle is heading towards him, and would be dazzled, even if still in the distance.
I try to focus on the white line on the right side of the road until the oncoming vehicle passes by.