Two weeks. No kidding. Chicks can handle adult chickens as long as they have a period where they can safely observe the adults and get to know their individual temperaments. After a week or two of watching the big girls operate, the chicks can start to mingle as long as they have a safe refuge to return to when they lose their nerve.
I call this refuge a panic room. It has chick-size openings the big girls can't fit through. Chicks' have their food and water inside so they aren't bullied away from these essentials. My first panic room was for three chicks to keep two adult hens from eating their food. I made the openings small enough, but I didn't make the panic room large enough. So one rather piggish hen shoved her head and shoulders through and got stuck. There I found her contentedly scarfing down all the chick food.
Later panic rooms had more space and more than one opening in case a panicked chick needed to get inside quickly. I make the openings about 4 1/2 in by 7 in. This accommodates chicks to ten weeks old. Go to my profile page and click on the articles page and then on the article about brooding outdoors. It has pictures.
