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at least 2m
500 to 1000 ft.
according to Florida handsbook, you must dim your bright headlights within 300 feet of the vehicle ahead.
Be considerate in using your high beams. Your headlights must be on low beam when you are within 500 feet (150 m) of an approaching vehicle, or within 200 feet (60 m) of a vehicle ahead of you, even if the vehicle ahead is in a different lane. You should also dim your lights for pedestrians approaching you. http://www.nydmv.state.ny.us/dmanual/chapter10-manual.htm (Alaska and Oregon are the same as New York - 500 feet.)
the courteous time to dim your headlights is as soon as you see another vehicle, or within one half mile.
Those laws vary from place to place. Common courtesy dictates to dim your lights well before they reach the oncoming vehicle.
If you are a courteous driver, you will dim them when you see their tail lights. No one likes bright lights in their mirrors, reflecting into their eyes.
1,000
500'
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.
You must dim your high-beams if you're within 500 feet of an approaching car and dim your high-beams within 300 feet of the vehicle you follow.
100 feet