Approximately 30-50'
Quote from FCTM:
When the threshold passes under the airplane nose and out of sight, shift the visual
sighting point to the far end of the runway. Shifting the visual sighting point assists
in controlling the pitch attitude during the flare. Maintaining a constant airspeed
and descent rate assists in determining the flare point. Initiate the flare when the
main gear is approximately 20 feet above the runway by increasing pitch attitude
approximately 2° - 3°. This slows the rate of descent.