The answer to how much diesel fuel a generator uses will vary according to how much power the diesel generator produces, what fraction of load it is being run at, and how long it is going to be run for. There are however assumptions that can be made.
The greater the power of a generator (usually measured in kW or kVA), the more fuel it will use per unit of time. The higher the load it is being run at, the great its fuel consumption will be. Naturally, the longer it will be run for, the more fuel it will use.
For accurate information regarding your specific generator’s fuel consumption at various loads, please contact your manufacturer.