Chicks are individuals with individual needs, and there are no "rules" to fit all cases.
There are some general guidelines you can use as a starting point. Most chicks are well feathered from four weeks and by six weeks they are usually pretty well insulated. During this period, it is a good idea to wean chicks off heat.
That said, you always need to watch your chicks' behavior and let them tell you what they need. If they are huddled in a tight bunch, apparently struggling to keep warm, then perhaps they need heat a bit longer. When chicks are avoiding the heat, getting as far away as possible, they nor longer need heat.
You can wean by decreasing the wattage from a 250 watt bulb to a 100 watt, and raising the lamp also can accomplish this.