Mine are nearing 4 weeks and have been outside for over a week now. A heat source the first couple nights and now they're put completely. Just so long as the night time temps stay in the high sixties,you use a heat source the first few nights, and they are mostly feathered they should be alright. Most people would say this is wrong but I'm speaking from experience.
As long as they're feathered our usually about 3 weeks, and the temps aren't super cold, and they have a good shelter, they should be fine. They'll huddle together to stay warm. If they get too cold, they may die, so if your temps drop too low you may want to stick your heat source back out there.