You don't have to ever switch them to layer feed. Layer feed has a flat rate calcium added to it and the chickens, which are very good at regulating how much they need, have no way of doing that with layer feed. Before they start laying, during molt, in the winter when they slow/stop laying, when they are sick/injured and not laying, when they slow/stop laying because of age, etc. etc. - in all those scenarios, they need less calcium than what's in layer feed, and continuing to eat layer will cause calcium buildup which is not good for them in the long run. That's why I prefer to let my chickens decide for themselves. I feed an all flock type of feed (no calcium added) year round and to all ages, and always have calcium on the side (crushed eggshells, you can use crushed oyster shell as well). I use Purina, and their all flock feed is the same protein % as chick starter/grower - 20% - so it's good for all ages. And makes life easier for you! I have never used layer feed and never will.