Poultry Talk Ontario (pto.ca)

I guess it depends on your setup, the mama hen, and your flock. Some mama hens are fearsome and will do an amazing job defending their chicks from the rest of the flock if the need arises, until they are old enough stick up for themselves.
In our setup we have a section of our run partitioned off by a fence, with a broody coop inside that section. We used to keep Gracie and her chicks in there for a week or so. If the chicks are well integrated and have known the flock since day one, they are usually fine by the time they are 6 weeks old, as long as they have plenty of space and places to hide if necessary. If you have chickens with bullying tendencies in your flock, results may vary and it might be wise to keep them "separate but together" (where they can see one another but with no direct access) a while longer. In our experience, the chicks were already fine to be together with the rest at 4 weeks, but mama and the chicks continued to use the broody coop until mam decided she was done mothering and went back into the main coop. The chicks typically went back to the broody coop for a few nights, followed mama into the main coop in less than a week.