200 to 300 feet
As I was taught, I do it as a courtesy upon sight of the tail lights.
Same with oncoming traffic – as soon as I see them.
However, the law here: approx. 150 feet (has not been updated since Jan. ’82 and lights are brighter now)
“265:114 Dimming Lights. – The driver of any vehicle upon approaching, overtaking or while following within a distance of approximately 150 feet of another vehicle on a way during the period from 1/2 hour after sunset to 1/2 hour before sunrise shall dim his headlights.”
The book answer in most locations is 500 feet. You don’t say where on Earth you are so that may not be the expected answer.
Everybody is right – actually do it when you see tail lights (or headlights from oncoming traffic) ahead.
if you can see their tail lights put your lights down to low beam
When you can see the tail lights.
See how it is with cars behind you.