As a general rule, you should buy what your dog can eat in one month. This may only mean a 10-pound bag for small dogs and a 100-pound bag for large dogs. It will depend on the size of your dog, how much he eats and the type of food you are feeding. Dogs often have to eat more of a lower quality food.
Ensure the expiration on the food is not expired.
For more information on Dog Nutrition, please read Nutrition in Dogs.