I usually go by the rule of one ounce per pound of weight. So, for example, a 30-35 pound dog should be drinking 4 cups of water a day. This is under perfect conditons though, not too cold, not too hot, no hard activity.
This doesnt apply to raw feeders though, thier dogs get most of thier water through the meat/veggies, which means for about every cup of meat/veggies, there is about 3/4 cup water.