Running out of my initial bag of chick starter and need to buy some more. Wondering how long I should be feeding the chicks the medicated chick starter. The person I bought them from indicated 6 weeks. The guy at the feed store said I could keep them on medicated feed for 4 months. What's the right answer? Also, the feed store near me sells Purina brand medicated chick starter. Is that fine or should I look for more options? Thanks!