Endurance for fighter is based on fuel consumption of the engine at full power at a specific altitude.
If you look at the the specification fora an engine they give the fuel consumption at so much grams, ounces or lbs per horse power per hour.
For example the 160 Ps (hp) Mercedes D.III data sheets gives the fuel consumption @ 230 grams or 230 / 1000 =.023kg per hour per hour and the oil consumption at 20 grams per Ps(hp)
You are the designer and you want your fighter to have a two hour flight duration at full power, it will require:
2hrs x .230kg x 160 ps = 73.6 kg fuel.
2 hr. x .02 kg x 160 = 6.4 kg. oil generally engineers use 1/10 of the fuel to determine the oil require or in the case 7.2 kg of oil.
The 150 hp Hispano-Suiza Model A engine has a fuel consumption of .52 lbs per hour hour per hp =
150 x .52 = 78 lbs.
Gasoline weighs 7.2 lbs per gallon, 78/ 7.2 = 10.83 gallons or gallons x 4.545 = liters, 10.83 x 4.545 = 49.2 liters per hour.
If the fuel tank capacity is known you can then determine the duration.
The Spad VII DATAFILE on page 24 , gives tha endurance as 1 1/2 hours, by dropping the rpm from 1450 to a cruise rpm of1300 would lower the fuel consumption, horse power and extend the length of duration.
An aside The Spad VII with the 150 hp Type 8A Hispano-Suiza was a marginal fighter aircraft, it was too heavy. The problem was solved when SPAD went to the 180 Hp Hispano-Suiza Type 8Ab engine, then it WAS a fighter!
P.S. I just got back from Vintage Aero Annual picnic. It was great! 11 P-51s 10 Ds and 1 H model. The Navy was well represented also, and a F-18 Hornet. It was well worth the drive.