My 04-06 Fazer takes 18 litres according to the manual (about 4,8 US gallons) and when commuting with alot of steady speed it goes around 250 km / 155 miles before the last fuel bar starts blinking... after that I could do another 40-50 km - but mostly I chicken out
The most I have filled up was about was 16,5 litres - after a total of 320 km.
There is quite a difference on how you fill up the tank. If you just fill up till the fuelgun clicks off then you easily still have a few litres of free space at the top of the fueltank. Try to fill it up to the bottom edge of the aluminum looking socket (that's full of drilled holes).