Mine stay on it for life I think as long as you offer oyster shell it doesn't matter. I haven't read the bag in quite awhile since I have used it for so many years. I guess I need too.On the FR crumble bag, it says after 18 week to change to Layena, that's what has me confused. I feed the Guinea Layena, but they do like the FR crumbles. I'm kinda confused as to what to buy next.