As a general rule, you should buy what your cat or cats can eat in one month. This may only mean a 10 – pound bag for single cat homes and a large bag for multi-cat homes. The size will depend on how many cats you have, how much each cat eats and the type of food you are feeding. It will also depend on if you supplement your cat's dry food with canned foods.
Ensure the expiration on the food is not expired.