One question is where are you located? What kind of temperatures are you seeing, either during the day when you take then out or the lows at night?
More importantly, where are they kept? Is the brooder outside where they see the temperature swings or are they inside a climate controlled house? What kind of conditions are they in now? Help us out a little bit so we know what your options are.
In general I suggest letting the chicks tell you what they need. In a ridiculous heat wave and brooding outside I once turned daytime heat off at 2 days and nighttime heat off at 5 days. Their behaviors told me they did not need it and they were much happier when I turned the heat off. In cooler weather I don't do that.
If they are hot they will get as far away from the heat as they can. If they are cold they will get as close to the heat as they can, usually in a group and giving a plaintive chirp. That is such a sad sound you know something is wrong. If it is right they scatter throughout the brooder. I brood outside and have a 3' x 6' brooder. I keep one end warm with a heat lamp but it's well enough ventilated that the far end can be 40 degrees or more cooler. Many people would be surprised how much time they spend in cooler temperatures, just going back to the heat when they need to warm up. A broody hen can raise them in snow doing this, just warming them up when they need it.