As long as chicks still have fuzzy down, they need heat. However, as they grow in feathers, they need less.
Even after the first week, when all they have are wing feathers, chicks are beginning to be able to hold in their body heat and only need occasional warmups under a heat source. By the time chicks are three weeks old, they have most of their feathers on their backs, sides, and underparts and they only need heat at night. In some warm climates, chicks at three weeks don't even need heat at night.
By the time chicks are four weeks old, they pretty much have all their feathers except for a few tufts of down on their heads, and they are pretty much done with needing heat unless it's close to freezing when your move them outside. Chicks at four weeks can handle 40s at night and 50s during the day with no heat after being acclimatized for a few days to the cooler temps.
