My lawn takes up 2 acres of my property and I have lots of stopping and backing around trees. I can go 12 to 13 hours on a tank with my B2620.
This is compared to my old Craftsman lawn tractor that was 4 to 5 gallons EVERY time I mowed.
I have pics posted on here too of some road building I have done. I pulled a 60" box blade up hill for 6.5 hours straight and still had lots of fuel left.
Old rule of thumb was a gallon/hour at rated rpm. I would imagine that you should burn something like 1/2 gal an hour while cutting. This is just a SWAG,however
Mine used $3.50 in fuel to mow two acres yesterday average fuel consumption is a little under 1/2 gallon an hour. It used to take 5 gallons of gas to do the same thing