It depends largely on the piston bore on the calipers and the pad wear, as that determines how much fluid is held by each caliper. The more worn the pads, the more fluid will be required to complete a full flush. That said, I've never worked on a system that required more than 3-4 pints so long as there weren't any bleeding issues. On the other end of the spectrum, I've had customers push 3 gallons through their system chasing air bubbles, but that's a different story.
FWIW, I flushed my '77 F150 the other day, disc/drum, with about two pints. I did a plugged port bleed on a new MC, connected the rear brakes and bleed until I got a good pedal and clean fluid, then connected the fronts and bleed until I had clean fluid at all corners.
