It depends on how you use your flash. f/8 ISO 100 is going to smoke your batteries compared to f/4 ISO 400. This makes a so called miles-per-gallon stamp very difficult. You can put your pedal to the metal and get crappy gas mileage or coast in idol all day and get great mileage.
As a general rule, I would say....long enough to worry about it just a little bit more than your camera itself.
1/60 shutter speed and 1/250 shutter speed, all else equal, with drain your batteries at the same rate.