Generators will use different amounts of gasoline per hour depending on the size of the generator and the load on the generator as well.
But for my generator it will run up to 5 hours on 1 gallon of gasoline which is pretty good so if I need to run the generator for 10 hours per day it would use 2 gallons of gasoline per 10 hours of run time.
Of course if I really put a load on the generator then it would consume a little bit more than 1 gallon of gasoline per hour.
Some larger generators may even use 2 gallons of gasoline per hour and the more load you put on the generator the more gasoline you will use.