I think OLD SMOKEY pretty much hit the nail on the head.And if the sector is a bit of both just log it as a bit of both.

Takeoff and Landing
 

I agree with all the comments to the effect that any reasonable estimate is sufficient to log time at night, especially after you have ticked enough boxes to collect all the licences you want. One requirement that does not necessarily go away (depending on your operating regime) is night currency for takeoff and landing. Even after many thousands of hours you will still want to know when you most recently performed a night takeoff or landing, so you need to count those. Of course, as a previous poster said, if you are logging the takeoff as night, you can fairly safely log the flight time as night as well.