If you're using the kind that is only calcium and does not have any vitamins in it, a shelf life of 5 years is totally fine -- that's how long we keep calcium carbonate on our shelves in my biochem lab before buying a new product, and I work with animals daily. Technically after 5 years you wouldn't have to get rid of it, but since you work with animals, it's not worth taking a chance of something being wrong with it (e.g. germs lurking -- anything that has been opened is subjected to them over time) If you are using the kind with vitamin supplements mixed in, I would stick to the recommended expiration, as vitamins can either be included in a metabolically active form, or a pre-metabolic form, and typically have lower shelf life.
