The lights on your vehicle must be turned on at any time day or night when persons and vehicles cannot be clearly seen for

1000 feet

The guidelines also include that:

1. The driver must dim his headlights when approaching vehicle comes within 500 feet.

2. When glared with oncoming vehicle, avoid looking into direct light and slow down.

3. Headlights must be turned on 30 minutes before sunset.

*It is also good to note that lower headlight beam makes it better for the driver to see through fog.