It will depend on the climate, whether they are indoor/outdoor, how quickly they feather out etc. The generally accepted wisdom is start at 95 degrees and lower the temp by 5 degrees each week until they have reached the ambient temperature of their environment. That said, I prefer to let my chicks lead the way vs. going by a set "rule" - meaning I provide heat while they indicate they need it and withdraw it when they demonstrate an ability to be comfortable without it. Our latest batch was taken off the light at about 4 weeks but were still inside at that point where the room temperature was around 70 - they were moved out a week later to their unheated coop where they enjoyed several days of 70/50 weather cycles and then an unseasonable cold snap that came with snow and temps into the 20s. They are happy, healthy and robust 8 weeks olds now.