How long does it take for the earth to revolve once on its axis?

One complete revolution of the Earth around the sun takes 365.25 days. A day is 24 hours, which is the time it takes the earth to revolve on it's axis plus the time it takes to catch up with the sun, which has moved 1/365.25 of one rotation away from where it was the day before. So the time for one rotation of the the Earth is one day minus 1/365.25th of a day, which is 23 hours, 56 minutes, 4.091 seconds. This is known as a Sidereal Day.