Your engine will use about 0.6 lb of fuel per horsepower an hour. Diesel is 7.13 lb per Gallon.
So with my boat's engine of 64 HP, it is: 0.6 x 64/7.13 will give my approximate fuel consumpion per hour of 5.39 gallons an hour. My tanks max is 180. I fill my tanks to 10% short of 180 which is 162 gallons and divide that with 5.39 gives me about 29 hours of operating time, considering the thievage of the height of the pickup tubes above the bottom of the tanks.
It is a good thing I have a ketch because the wind is free.