Hey, depends on your climate and where you live. In warmer climates, south, they can feather rather quickly. Best to give enough time and feathers to avoid night time temps that could chill them along with other conditions such as rain, especially if they can get a little damp.
6 weeks is safer, less than that, consider the above, We were able to put ours in the coop at 4 weeks, they were Easter incubator babies.
We had 2 hens go broody, those 2 week old chicks were fine with their Mom outside in the peeper pen/coop. But, 4 days ago, hens left the nest, too crowded. Those peeps did well 4 or 5 days in our warm Gulf Coast weather, until these big rains and lower temps came in on Wednesday.
I brought them back inside our outdoor porch bathroom with a 75 watt light bulb on them. They will stay there most likely for at least 2 weeks, until they are feathered out and our weather is warmer/stable.
Hope this helps.
