500'
when you see there lights you should dim yours
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 law states you dim your lights when the approaching vehicle is within 500ft or if you are following a vehicle within 500 ft. It's in the commercial drivers handbook from the DMV. I assume the same info is available in the non commercial handbook but I don't have one to check.
When you are within 500 feet of an oncoming vehicle or you come up behind a vehicle.
Those laws vary from place to place. Common courtesy dictates to dim your lights well before they reach the oncoming vehicle.
1,000
at least 2m
You usually cannot have your highs on within 200-250 feet of an oncoming vehicle. Essentially, if you can see any kind of glare from their headlights, you should disable your highs. You should not be driving with your high beams on unless on a country road where no light posts are installed or during inclement weather where visibility is less than 5 miles.
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.
Railroad tracks, tunnels, or intersections.
according to Florida handsbook, you must dim your bright headlights within 300 feet of the vehicle ahead.
You should dim your lights as soon as you see their lights.