It will depend on how often your system runs. The diesel burner will heat up the boiler to the deignated temperature then shut off. The circulating pumps will circulate the hot antifreeze to the heat exchangers throughout the coach in response to the thermostat. As soon as the boiler cools down to a certain temperature the diesel burner kicks in again.
How often this happens will vary with how cold it is outside, how much area your coach has to heat, insulation levels, etc.
If it had to run full bore the burn rate is .35 GPH so you could run 3 hours on 1 gallon of diesel fuel. However, with the cyling of the burner due to the boiler's high limit cutoff switch, I've never seen mine run at 100% even in sub-zero temperatures.
I do find a huge advantage in cool weather camping with the HydroHot compared to propane furnaces. 32 gallons of propane just doesn't stretch as far as 150 gallons of higher BTU diesel fuel. I don't miss running for LP any more like I used to.