The whole point of medicated feed is to suppress the growth and reproduction of the Coccidia parasite, which can be found in almost any soil in the world. Young chicks are not born with natural immunity of it, so the medicine in the feed slows down the parasite, which allows the chick to build it's immunity up without being overwhelmed.
With this in mind, I feed my chicks on medicated feed all throughout thier brooding, and for at least a week after they go outside. Afterwards you can just switch over to regular pullet developer up until they lay, then switch to laying feed.
I hope that helps clarify things for you, it's a method that works really well for myself, but there is no set rule and many people have good results from different methods too.