If you can see the area lit by their headlights, then you are probably close enough to dim your own.
Common courtesy would be at least 500 feet.
300 feet
According to the California Department of Motor Vehicles, the minimum following distance on highways is three car lengths. Anything closer is considered tailgatinging and is both unsafe and against the law. This distance is independent of time of day.
200 Feet
In most states it is 500 Feet. Common sense should prevail.
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.
What is following distance? Following distance is the space between your vehicle and the vehicle in front of you while driving. It's important for all drivers to maintain a safe following distance, so that they have time to safely stop in case the vehicle in front of them brakes abruptly.
never
when you can see their headlights
Pay attention, allow enough stopping distance when following another vehicle.
the courteous time to dim your headlights is as soon as you see another vehicle, or within one half mile.
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.