You'll have to refuel at lunch plowing but should go all day with a 9-ft disc. If you are discing light enough to pull in 3rd you could go all day with a 10 ft disc too.
Check the Nebraska tests for hp-hr/gal, then assume 35 hp pulling it's guts out and 20 hp under light load. It is hard to get the ground conditions right to actually consume 35 hp with an M.
Fuel consumption will normally range from 1.5 gal per hr unloaded to 3.5 gal per hr fully loaded.