My pup would eat a whole bag if I went by how he acts (or would try). Go by your puppy's body condition to determine how much to feed. If your pup is nice and lean, not emaciated, not fat, with maybe a bit of rib showing and a waist when viewed from above, you're probably feeding a good amount. Up or lower the amount by a 1/4 cup if your pup starts to get pudgy or begins to look skinny (all ribs showing, hip bones jutting). I determine how much I feed every day depending on what my pup looks like.
Xan Varik von Fernheim (born 4/3/13)