I got 360 miles from a £65 tank last year and believe it or not that was my Omega work car, its a 2.5V6 always loaded with tools and this time 2 people and 2 peoples luggage, and im not miss Daisy either lol.... i got a fuel filter to fit to that and il be adding some system cleaner as its not been done in 5 years, due to this i think i should notice some difference.
As far as using a cleaner that often i think it be a waste of money, there wont be any deposits to be cleaned out so no point using it, i would think if you do high mileage then every 6 months may be usefull, otherwise maybe once a year?