How long does it take for a day to pass on Neptune?

Neptune's sidereal rotation period is 16 hours 6 minutes 36 seconds.

I can't find the precise length of a solar day there, but since Neptune's year is so long it would be only very slightly longer than that. (On Earth, the difference between the sidereal rotation period and the solar day is about 4 minutes; Neptune's year is 165 times longer, so the difference between the sidereal and solar days would be at most a couple of seconds.)