We all have different weather and conditions. I really hate trying to give magic numbers for chickens, whether age, space, temperature, just about anything because we keep them in so many different conditions. But I know people that don’t have a lot of experience need a starting place so I will give numbers, much as I hate to.
I raise my chicks in a brooder in the coop. Last summer, in our sweltering heat, I turned the daytime heat off at 2 days. I turned the overnight heat off at 5 days. Days, not weeks. After you get used to them, they’ll tell you what they need if you pay attention.
I’ve had broodies take chicks to the roosts at 2 weeks old where they slept with absolutely nothing to help keep them warm. This was in the summer and it was pretty warm. I don’t remember the overnight temperatures though. I’d guess in the very low 70’s.
Last fall, I kept the day and night heat on them until they were 5 weeks old. Then I moved them to an unheated grow-out coop. There were about 20 in this batch. The nighttime temperatures were getting down into the mid 40’s Fahrenheit. I kept one end of the brooder heated but the rest cooled off a lot. I really think them playing in that cooler end helped acclimate them.
The grow-out coop had really good draft protection and good ventilation up high. When they were 5-1/2 weeks old the overnight low hot the mid 20’s F. Those chicks were fine. If they had not been used to cooler temperatures in the brooder, I’m not sure they would have done as well.
To me, the ideal situation is to give them a warm spot to go to, then give them the option of spending a lot of time in a much cooler area. Let them decide where they want to be. With the way I raise mine, I would have absolutely no problem with them spending hours at 70 degrees at age 4 to 5 weeks. Mine are often in the grow-out coop in the summer at 4 weeks and the nights can easily get into the 60’s. I never provide heat after 5 weeks, even in colder temperatures.
But you don’t raise them the way I do. Until you get some experience, you probably should be a bit more cautious.