I put ours out when they started to drive us crazy (the brooder resided in the livingroom, which is by the bedrooms). I think it was about 4 weeks - they had most of their new little feathers - not big by any means, and they were wee little things, but the coop is solid and draft free. We had the heat lamp in with them until it started getting warmer out (above 70 in the day). We kept them in the coop for about a week, and limited outside in the run for another week. They now have full access to coop and run (both are pretty darn predator proof, completely enclosed), and at 10 weeks now, they are pretty happy and heathly.
