How long does it take to rotate its axis?

Answer 1

Neptune takes 16 hours, 6 minutes and 36 seconds to rotate once on its axis. That's about 2/3rds of an Earth Day.

Note that the period of rotation is measured by watching Neptune's magnetic field. The time cited is the time it takes the "whole planet" to rotate once on its axis. This is important because Neptune is not solid - it's a big gas planet, and the atmosphere is rotating at different speeds.