It depends. When I did night flights to Europe, the lights stayed on. There is nothing to see over the North Atlantic at night and it helps keep you awake. Now I do flights up to 16 hours long during the night and day and sometimes night again! The lights are on at night during cruise. The lights are out in my bunk when I'm sleeping.

When I did short domestic trips (like transcons) I could handle lights out for a few hours.