I use full tank diesel per week
Prior to hot water install it lasted 10 working days
I calculate 1.25 litre an hour usage.
I think the fuel figures of a litre per hour from webasto and others are a little too keen.
However if you work alone, have another pump fitted and keep that on all day, by 12 noon you can turn off burner as the re circ feature would have heated all your water in main tank.
Mr B