Every time I test drive a brand new car they typically have 5-8 miles on the odometer and the gas is usually very low (mostly seems to be an 8th of a tank or less).
The last car I test drove brand new was a 2007 Pontiac Grand Prix and it had about an 8th of a tank. The Grand Prix holds about 17 gallons so that'd mean GM put 2-3 gallons or so into the car to get it out of the factory, onto the truck, and around the dealership. I imagine that amount might change depending on the vehicle and its engine.
In any case, I always insist of a full tank of gas before I leave the dealership. The salesmen like to whine about already not making any money on the car but don't give in.