From the time chicks hatch, they require different levels of heat as they age. Day-old and week-old chicks need temperatures to be no less that 95°F.
As the chicks get older, the temperature of the brooder box needs to be decreased five degrees each week they age. For instance, chicks that are two weeks old need a temperature of 90°F, and chicks that are three weeks old need 85°F.
Decreasing the temperatures in five degree increments continues until the chicks are fully feathered and can sustain the temperature at which the brooder box is located.
On average, chicks that are inside a house (65°F-70°F) can be without a heat source when they are six to seven weeks old.
It varies if the chicks are in a brooder box outside. Temperatures get colder at night, so the chicks will require artificial heat for longer than it would if chicks were inside a house. If so, the heat source will continue to decrease five degrees until the temperature of the brooder box can match the temperature outside.
After the chicks are fully feathered, I normally keep them inside without artificial heat for an additional two to four weeks. It depends on the season and weather conditions. If it is warm, as in summer, I usually take them outside when they are nine weeks old. If it is colder, I wait as long as possible to transfer them outside. I want to ensure the chicks take well to the change in temperatures.
If the chicks are fully feathered and the temperature outside is the same as inside of a house, chicks can go on "field trips" outside. "Field trips" are when chicks go out in a safe and enclosed area for a couple of hours. This will allow the chicks to become acclimated to the conditions outside of the brooder box. I usually set up a cage in a sunny area with access to shade, food, and water. After a couple of hours, I bring the chicks back to the brooder box inside and do repeat "field trips" if the weather permits and is warm enough. I do not recommend "field trips" on rainy days. I would accompany the chicks on the first "field trip" to see how they do.
Once the chicks are old enough and can sustain the temperatures outside, I transfer them out into grow-out cages.
*Below is an example of how the temperatures change as the chicks age. To get decreased temperatures in a brooder box, raise the heating source and use a thermometer to measure the temperature.
Age Temperature
Week 1 95°F
Week 2 90°F
Week 3 85°F
Week 4 80°F
Week 5 75°F
Week 6 70°F
Week 7 65°F
Week 8 60°F
Week 9 55°F
Week 10 50°F