The longest non-stop scheduled airline propeller flight was....
20 hours 25 minutes, average, depending on winds.
Operated by AirFrance using Lockheed 1649A Constellation equipment.
Absolute endurance for this aircraft was 23 hours, provided the oil was not depleted.
Fuel capacity for the 1649 was 9817 US gallons, oil 221 US gallons.
A very nice aeroplane to fly. Personally operated these same aircraft in freighter configuration after they were sold by AirFrance.