When should headlights be used?

Answer 1

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.