Fuel goes bad quickly in small quantities but not in large quantities (as quickly, as small quantities) like in your fuel
tank. It takes much longer for the gas to go bad in the tank verses the carburetor bowl. (but the fuel in the tank will go bad, read below)
I would suggest fill the tank full with hi test (hi octane) and put fuel stabilizer (I use marine fuel stabilzer because it
helps with condensation build up in the fuel tank) in the gas you get from the pump. Run the generator for at least 20 to 30 minutes every month or two. If the fuel in the tank drops below 1/2
tank, fill it back up with hi test (put stabilizer in fuel from pump) . If you do not go through a half tank of fuel in a 24 month period, I would suggest running the fuel down to at least half
or less (you can drain it or run machine) then refill to full. Repeat as necessary.