You begin weaning off heat by the end of the first week by simply raising the lamp so the temperature of the heat "footprint" is a few degrees cooler. There is no set rule book for the amount of heat. You generally go by the behavior of the chicks.
As chicks grow real feathers, they are beginning to help insulate the chick against lost body heat, so you should see your chicks visiting the heat zone less frequently. They are, in fact, weaning themselves off heat.
Generally, by the time chicks are three weeks old, they no longer require heat during the day when the ambient temp is mild (65-70F). Their new feathers prevent heat loss while at the same time, they are eating all day long generating heat from food calories. Thus, an eternal heat source becomes less of a necessity.
They will require some heat at night until they completely feather out around five or six weeks. The reason why they need heat at night isn't that it's colder, but for the simple reason they generate less heat from calories while sleeping instead of eating. Once they have complete feather coverage, they are well insulated against losing body heat, even on cold nights.
By age six weeks, if properly acclimatized, chicks can move into the coop without heat as long as the ambient temps are above freezing.
