3-4 weeks depending on your weather. I removed the heat lamp from my tractor when they were 3 weeks, and they've done fine. Night temps where in the 50's when I removed lamp. If it is too cold for them, they will pile up and you'll know they still need heat.
3 weeks is usual here as well. I pulled the heat lamps and two nights later the over night low dropped below freezing. I ended up running heat lamps for 4 1/2 weeks this time.
Usually I put them in coop at 4 weeks with a heat lamp. However I still have them in brooder this year at 5 weeks due to night time temp still being down to 36 this year. It has been an unusally cold year.