From what I've read, it's your choice when to switch them off of it. Some people don't even use medicated starter. They need to stay on medicated or non medicated starter until they mature...I think I remember it being somewhere between 14-18 weeks when you switch to regular chicken food. But to switch to regular starter from medicated...that's up to you. Medicated does NOT protect them completely from cocci, it just gives them a better chance of not becoming infected.
Edited to add: this is what I was told by the lady at Southern States regarding the cocci.