I don't know that there are any figures freely available for generator fuel burn (if there are I wish I could find them).
However, if you consider that a typical generator consumes about 150-200hp at full power, and that a typical aircraft engine is making well over 50,000hp (depending how the number is arrived at - a whole other topic) you can see that the fuel used to turn the generator is really rather small, something in the region of 0.3% perhaps?
Then take into account that most generators are only running at 30-50% load during cruise, so essentially 0.15% of your fuel flow.
An APU is about 15-20% efficient at turning fuel into electricity, and gobbles up anything upwards of 120kg per hour for a 50kVA unit.
Maybe you can do the maths for your engine and work out the fuel flow required to spin the generator...