Any time after six o clock
Laws will vary a bit between jurisdictions, but this version from the State Of New Jersey is pretty common;
"Proper use of headlights is critical to safe driving. Headlights must be used between one-half hour after sunset and one-half hour before sunrise. Headlights must also be used when visibility is 500 feet or less, when using windshield wipers (during rain, snow and ice) or when encountering fog, mist, smoke or other factors that reduce visibility."
Note that many semi trucks will use their headlights 24/7. It's a safety issue. Being seen is important.
How far should headlights shine?
Concave mirrors are used in headlights.
No. Either use your regular headlights since they help other drivers see you. Also, slow down.
Headlights need to be used at night, when it is dark, and when windshield wipers need to be used.
Headlights are used during times in which it may be hard to see. In most states, headlights must be used when driving from dusk before sunset to sunrise.
300 feet
When it is raining during the day you should always keep your headlights on.
What Should You Do - 2003 Deer in the Headlights was released on: USA: 14 August 2004
At night time or 30 min after sunset In Illinois they are also required when wipers are in use.
All the time
Wipers
Deacrese