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

It depends. The ‘sidereal day’ is 23 hrs 56 mins (and a few seconds) - and that is the time for the stars to appear to revolve around us (remember that the stars don’t actually move around us like they appear to!!) ...so imagine looking up at a star in the sky and spending a day watching it and timing it travel a circle and go back to where it started from. That would take 23hrs 56minutes. But 24 full hours is the time we measure as one day on our clocks.