I usually fill my tank every two weeks.
I do not do a lot of driving as I love spending time at home and even though I have a car with GPS I still fear getting lost.
My tank holds perhaps 50 litres though that is a guess as I really do not know! It is a medium sized car so perhaps it holds more than that.
I will also not buy discounted supermarket petrol as do not want to support them in this venture.