Three thoughts:
1) Why not think of the range provided by the fuel? Fuel is usually used to go somewhere, the fuel amount is usually resolved as a range, not a time. 3 hours at 60 knots might be the same as 2 hours at 90 knots.
2) Fuel tanks weigh about .5 to 1.0 lbs per gallon, so each additional gallon of gas allows less payload, even if the tank is not full.
3) Human endurance - I am good for about 2.5 before I need to land, sorry!