I have no idea where you live, not even which continent. I have no idea what your weather or temperatures are like. I don’t know what your coop is like or if you have adult chickens in the coop. I cannot speak to your unique situation.
My brooder is in the coop to start with and it is big enough that I only heat one area. The rest cools down as it will. That way I only have to keep one area warm enough and they can get away from the heat if they get too warm.
I have kept the heat on for five weeks, say when the overnight temperatures are around freezing or below. I’ve moved 5 week olds to an unheated grow-out coop with overnight temperatures in the mid 40’s Fahrenheit. In our triple digit heat wave last summer I turned the daytime heat off at 2 days and the overnight temperatures off at 5 days. Days, not weeks.
There is a whole lot more involved than just pure age. But if you don’t have adult chickens out there, you could probably have had them out there from Day 1.