I guess I am out of the norm here. My goal is to raise free range chickens so I work towards that as quickly as possible. My first bunch when into the coop with a heat lamp at 3 weeks old. At this point I had already been letting them out to forage for several hours a day with supervision. After they spent about a week in the coop I started letting them out to forage on their own. They mostly stayed in and around the coop which is in a covered barn. As they got older they slowly started venturing out. They are about 13 weeks old now and free range full time when we are home and have a run for when we are not. We added two full grown RIR when the youngins were about 6 weeks old. They integrated peacefully and the older ones had no problem venturing to our tree line to forage and it didn't take long for the babies to figure out that they could get the best pickings in the composting leaves at the tree line. I have added 2 more times to that established flock, keep them in the coop for a week and then let them out full time. Yes, I have lost a few to predators, but if I am not free ranging I am defeating the purpose for which I got them. Free range organic eggs.
I recently purchased week olds and they have spent at least an hour a day outside foraging. I do my best to replicate, at least in part, how it would be if they had a mother hen to take them out. I also, after researching, have come to believe that being out, even at a young age, develops immunities and gives them "grit" via the dirt they pick up when they eat. You should see the excitement of a 2 week old chick who catches her first bug! She goes crazy and just runs around in circles. Even at a young age they love getting out there and scratching around, sun bathing and even having a dust bath. I watched 2 of them try to push each other out of a little hole they had dug and wanted to sleep in for nearly an hour yesterday. So funny!