At least 8 weeks, but if you can free range them together for some "bonding" during that time, the transition is better. When they're small like that, it only takes one good peck to seriously hurt them or worse.
My broody over the summer had a heck of a time. She started with duck eggs. 25 days she had them, before the broody duck screwed up hers. I gave the chicken's duck eggs to the duck. She sat on an empty nest for a couple of days before I broke down and bought her some chicks. Then the other hens were jealous or something, so I moved her. She was so happy after the move and having those chicks. Like all her stress was gone. The other hens were either trying to get at the babies, or use her nest box.
Now she's depressed, full blown half naked molt, and just broke from being broody again. I didn't give her any more babies. Once a year is enough. She's so thin! She's on an extra special diet now.
When I let mom and babies out to range, her best friend came charging over and hopped into their hut. Ate their food, scoped out the nest, and visited with the babies. The others came by and looked too. They accepted those babies as hers, no meanness, not like when she was in the coop.
I wonder if it's instinctual. Hens disappear from the flock, and return with babies. Maybe the hen needs to disappear from the flock to know what's happening and accept it. LOL Cooped birds just don't get it?
When she started taking the babies back to the main coop on her own, that's when I moved them back in. They were feathered out when she was comfortable doing that, about 7 weeks I think.
It got a little stressful for them when momma decided she wasn't protecting them anymore. When they're feathered out it offers some protection, and they can run fast. At that age they become paranoid and distrustful of other hens.
One neat idea is to make a wall with slats... think of a baby crib. The babies can fit through it, but the adults can't. Put the starter feed and some water in there. It's like their retreat to hide in.
