As soon as the oncoming car is in sight.
when you see there lights you should dim yours
500'
You should dim your lights as soon as you see their lights.
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.
to dim the lights in your car.
This distance may vary according to your particular states (or country's) laws. As a general "good driving" rule - as soon as you see their headlights approaching, you should dim yours.
If your interior lights are dimming, either your battery or starter could be failing. You should have your car checked.
If you can see his headlights, drop your high beams. In some states its the law 300 feet at least.
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.)
500 feet is the law in most states. So imagine 2 football or soccer fields. Or one of each!
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.
Flat battery !