How much gas depends on how you use your generator, and whether it is a sipper like a Honda 2k, or a guzzler like a Champion.
Keeping a record of your battery voltage will determine how much you need to replace with your generator. Ideally, you will use the middle portion of your battery capacity, and replace only that part on a daily basis. That might take an hour of generator, or maybe two on a daily basis.
Last year I went to the neighbor and asked him why he was running his generator. He was charging a cell phone. I showed him my 75 watt inverter, and said it would charge cell phones quietly from the battery.
I might use a gallon of gas in a week of dry camping, maybe only half that, but I am vey conscious of what my batteries need.