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 when an oncoming vehicle is about 500 feet away. This helps avoid blinding the other driver and ensures safe visibility for both vehicles. Additionally, if you are approaching a vehicle from behind, you should dim your lights when you are within 300 feet.
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.
When approaching a car using high-beam lights, you should dim your headlights to low beam to avoid blinding the oncoming driver. This is not only courteous but also a traffic safety regulation in many areas. If the other vehicle does not dim their lights, maintain your focus on the road ahead to avoid glare. Always be aware of your surroundings and adjust your speed accordingly.
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!