Don't know the specifics of the Audi Q7 of the OP, but my Mercedes A 180 CDI 2012 model (Diesel engine) averages 53 mpg (4.4 l/100Km) for my conmute trip (35 KM total, including ~8 KM of mountain road, with a 300m elevation change). Given that my A180 has a 50 liter (13.21 US gallons) fuel tank according to the manufacturer, that means I could make up to ~1,100 Km with a single tank. In practice I refuel before that, but routinely make ~900 Km between refuels.