the general rule is wait till they are fully feathered. By then, they should be pretty much "weaned" off the heat lamp. have to tell you...I tossed my last batch of babies out early. They stunk, they kept getting out of the brooder, everything was covered in chicken dust. I was sick of cleaning baby poo off of everything. They hadn't quite gotten all of their feathers in on their heads and necks, and it was pretty cold (colder than 60). I put them in the little run, stuffed it with straw, like REALLY stuffed it with straw to help with heat retention, and gave them a dog carrier so they could all pile in and keep eachother warm. I do not have a way to provide a heat lamp or any other heat source out there.
They did just fine. They quickly learned that by going inside the carrier, their body heat kept it nice and warm. As long as you give them access to an area that is nice and warm, at eight weeks (I think I put mine out at six or seven weeks), they should be okay. Just keep an eye on them, and if they are acting like they are truely cold and miserable (not just complaining about it!), make adjustments accordingly.