The simplest answer is as much as your dog wants. A dog should always have a supply of fresh water available, and if this is not possible then they should be supplied with fresh water at the very least 3 times a day. There are various calculations such as 50-60 ml of water per each kilogram of body weight or 3-4 times the mass of solid food intake with which to calculate your dog's exact requirement. However your dog should be the one to regulate his intake as every dog is different and the amount required also depends on the type of food they eat, the amount of exercise they do and the temperature of their environment.