This will vary per breed of dog. Are we speaking about a Chiwawa or a German Shepherd? Is your dog a "lay in the sun" or a working breed or service dog? Just as you would drink more of a sports drink after a workout versus a day in which you are hanging around, so will your dog after a long walk.
A dog should drink one ounce of fluid per pound each day. If your dog is twenty pounds, he should consume twenty ounces each day.
Dr. Jill Stefanelli states that "working breeds and service dogs are clearly not being hydrated adequately daily. Hydration is directly proportional to the amount of work a dog performs daily."Joey Bee