When I was researching this myself last year, a couple of sources that I read suggested that medicated feed be fed until 14 weeks to obtain the best protection. Now, that said, feed manufacturers often suggest that grower be started at 8 weeks. For me, if I had the option (I didn't have grower in my area) I'd probably go with the medicated feed longer to be sure that I covered all of my bases in that regard. With no grower in my area, I kept them on medicated feed until 17 weeks when I began switching slowly to lay feed. This had me switching to lay feed earlier than I would have liked, but I wanted them off of medicated feed before the first egg was laid.