We keep them in their new coop for 3 - 4 days, starting at 6 weeks - this lets them know where they "live" now. After that we let them out supervised in the late evening, about 30 - 45 minutes before chickens normally go into roost. They usually all go back inside when they are supposed to, but there are usually a few stragglers who won't go into the coop the first couple of times. We do that for 3 or 4 days. If everything goes well, we let them out earlier in the evening, unsupervised, for a week or so.
If the adult chickens (who are also free ranging, but live in a different coop that is about 500' away from the "new bird coop") have not been picking on the new birds, then by 8 weeks the new chicks let out in the morning and left to their own devices.