Yes, headlights must be on low beam within 500 feet of an approaching vehicle.
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.)
When approaching another vehicle from the rear at night, you must dim your high beam headlights within 200 to 300 feet of the vehicle. This helps prevent blinding the driver of the vehicle ahead and maintains safe driving conditions for everyone on the road. Always be mindful of your surroundings and adjust your headlights accordingly.
at least 2m
500 to 1000 ft.
Those laws vary from place to place. Common courtesy dictates to dim your lights well before they reach the oncoming vehicle.
1,000
That you adjust your speed so you can react and stop within the distance lighted by your headlights.
500'
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.
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.