14 CFR §91.151

1. (a)No person may begin a flight in an airplane under VFR conditions unless (considering wind and forecast weather conditions) there is enough fuel to fly to the first point of intended landing and, assuming normal cruising speed—
   1. (1)During the day, to fly after that for at least 30 minutes; or
   2. (2)At night, to fly after that for at least 45 minutes.
2. (b)No person may begin a flight in a rotorcraft under VFR conditions unless (considering wind and forecast weather conditions) there is enough fuel to fly to the first point of intended landing and, assuming normal cruising speed, to fly after that for at least 20 minutes.