How much fuel does our engine use when idling?

Looking back on our 2005 hurricane evacaution where we spent half a day idling and crawling, I think I used about half a gallon per hour (or perhaps just slightly more) in a 2.7L V6 engine in my S4. An engine twice the size seems like it should use about twice as much, which would work out to your estimate of about a gallon per hour.