The best place to brood chicks is right in their coop.  This assures that they have plenty of room, (by the time they are 2 - 3 weeks old, they should have about 2 s.f. of open space per chick) saves them the stress of moving from brooder to coop (chickens hate change) and if given enough space they will wean themselves off heat.  Many of us have made the switch to brooding in the coop (even if we already have adult chickens) and use a heating pad cave instead of a heat lamp.  It is much safer, more natural, and mimics the heat provided by a broody hen.  Chicks should be completely weaned off heat by 4 - 5 weeks of age, depending on the season. (I'd give them longer if it was early spring, late fall, or winter) In the summer, based on ambient temps and number of chicks, you could plan on having your chicks completely off heat by 3 weeks, and even sooner if you use a wool hen or a huddle box.