Years ago (and I believe I do have this information filed....) someone did a study and if I remember correctly, they calculated : 1 hour = 60 miles on the truck. This does make sense since the motor is still running.
I was thinking the question was asked in regard to fuel use and tax rebates, and now realize that it can also be important to set maintenance points.
For tax rebates, we put fuel meters on several trucks to tell exactly how much fuel was used by the PTO. Fuel use was less than what was quoted by the manufacturer, so we were better off using the higher figure.
For maintenance we've used the manufacturer's figures combined with our use. Plugs and wires get changed at 25k intervals, transmissions get flushed at 50k, and oil filters get changed every month. No other special maintenance needed.