I think you can sometimes harden them off a little sooner depending on the weather and brooder. What heat source you use could be a consideration too. Heat lamps don't offer much residual heat. An electric hen will stay warm for up to 30 mins after it's switched off.
We have some 3 week old chicks in a 4ftx4ft brooder in the shed. It's fully draft proof with a 4x4 run that's side boarded to prevent drafts. They are off the heat during the day. Only putting it on at 10pm till 6am. This nice weather is helping, if the temperature drops I might give them a bit of daytime heat for 1 more week, but I'm expecting to have them totally off the heat by 4 weeks. In the depths of winter though it's another matter! They'll soon start chirping very loudly if they are cold.