I think it really depends on how many you have, and where you are. I used the lamp to heat up a wall of the brooder from the outside for the first 3 weeks, and as time progressed, they used that wall less and less. After 3 weeks my 8 birds seemed to not need it anymore, which was when I removed the lamp. Nights were mostly around 75, with the lowest being 69 (iirc), they seemed to keep each other warm well enough. I think brooder size will affect heat retention too. I‘d say watch your birds’ behavior and proceed to make little adjustments from there.
Im a first time owner though, so you should probably take my experience with a grain of salt.
Im a first time owner though, so you should probably take my experience with a grain of salt.