That depends on what your feed plan is. Do you intend to feed a grower ration as an intermediate, or are you planning to make the change directly to layer ration? If you are going to feed grower, the change can take place now, could have been done weeks ago - if you are going to go straight to layer ration you will need to wait until your birds start to lay or at least 18 weeks of age. Another option is never - many folks (myself included) prefer to feed a grower ration from day one to the last day of our birds' lives.....this makes feeding a mixed flock (actively laying and birds not laying due to gender, age, molt, broodiness, etc.) ridiculously simple as all you need to do is provide free choice oyster shell for those birds who are actively laying to take in to meet their need for calcium and the actual feed provides appropriate nutrition for all the birds in the group.