I don't remember the actual number of weeks ... but at ... six to eight weeks start switching from chick to starter/grower feed. Then, at about twelve to fourteen weeks, start switching to layer feed. Don't mix in any ouster shells ... leave that separately available to them as a 'free choice'. They'll eat it as they need it.how long do i need to wait too change my chicks food over to adult food??? and do you mix oyster shell in with the adult food when they make the change???