My haynes manual quotes 89 litres for the fuel tank, and i've had nearly that put in it when i bought the thing. chap sold it completely dry.
My 97 Disco usually gets anything from 24-29mpg with mainly very short journeys, most of my journeys are roughly 5 or 6 miles, on a run it can be pretty good though, but when you average it over the tank its about 25odd mpg. basically about 450-500 miles to a full tank