The answer is once.
The dogs' digestive system is set up to eat only once a day. Canine stomach acid is much stronger than that of a human, hence they can digest bone etc., however putting anything in it, reduces it's strength and only time with an empty stomach can bring it back to it's full pH.
Therefore feeding more than once a day, or providing the ability to graze, actually reduces the dogs ability to digest what it is being fed. With the gastro-intestinal tract being 80% of the immune system, multiple daily feeding has a negative effect on the immune system, increasing the likelyhood of illness.
Therefore feed your dog once a day, the occassional treat is fine, and a weekly starve day will help boost the digestive and immune system.