How longs a piece of string?
Depends on a lot of things. Driving style, tyres, load, road conditions, traffic congestion etc...
If I fill up with 45 litres I can get between 280-300 miles, and it varies from 29mpg to 34mpg.
Edited to add - I've got road biased tyres on which have helped a huge amount. Recently put my roof rack rack on but I doubt this makes much difference to fuel consumption, its built like a brick anyway.
If the engines looked after it'll help. A snorkel will decrease mpg a little (I have one fitted). Also remember that the type of driving I.e. motorway, urban etc... Will make a difference too. Much like any car