How much? I'd rather throw a few litres of ancient crud than pay that all the time.
Right now my generator is being used on a monthly basis, so I'm not too worried about fuel aging, but it is worth bearing in mind if you don't use yours regularly. If nothing else, keep the generator tank close to empty, and if you can, cycle the contents of the jerry can into your car every 3-6 months and refill.
I've found no problem using petrol from a full, sealed container up to a year old.
It's not just the fuel aging thats the problem, the oil seperates and gumms up the carb and fuel lines and its then the time and know how for some people to be able to strip and clean out the carb etc.
------------- Doing as little as possible for as much as possible...
Not that I use a genny but my advice is when you've finished using it at the end of the season empty out the tank & fire it up on the carb full of fuel thats left till the motor stops. That way none of the seals, diaphrams etc will get damaged or gummed up by the fuel. I do the same with my lawn mower & my brush cutter when I've finished with it for the year. Soon as I put fresh fuel in the following year a couple of tugs on the cord & we're away.
Amazing family weekend with old steam engines, classic car displays, market stalls, and full catering and bar. And camping on site - Save £25 by booking in advance.