I really don't know how long. I've heard different ages for the medicated feed. This is the first time I ever used chick starter since I'd always just let Mama Bird decide what her little peeps needed. If you buy organic chicken that means they are not given any chemical products. Instead they use methods like oregano in the water. I'm pretty sure most commercial poultry houses use medicated feed in their houses so if you've ate regular (not organic) chicken, you've probably ate a medicated chicken.
I can't say my chicks were any healthier with the chick starter than my one raised by real mama birds. I have a little leftover if I get some more chicks but otherwise I feed mixed grains (occasionally) and leftover kitchen scraps to my birds. I think from now on I will try to find non-medicated feed because one of the reasons I raise my own birds is to know my meat and eggs is non-medicated. My chickens are always running by with a frog, bug, small snake, mouse, etc. in their mouth and eat grasses and seeds from grasses. I'm sure they are healthy and I know they are happy chickens.
They also free-range during the day-light hours. I do give my birds more grain in the winter and they free-range much less. I've been lucky not to have any sick birds. One died of old age, I gave a lot away when I got too many. Maybe you can find a middle ground for example: one month of medicated chick starter or even buy non-medicated kind.