It's great seeing all the different opinions on the matter...
This is what I do:
They get is 24/7 for the first week of life. After that, I pull the lamp up a little bit each day to gradually wean them off of it. After that, I start shutting it off during the day, and letting them have it at night. Then they dont need a light by 2-3 weeks old. If they are in the house, I wouldnt worry about it too much..My house stays around 60 on a warm day, and it isnt drafty where their brooder is. Unless they are huddling so much that you are worried anyway...Mine are only 2 weeks, and they told me their 60watt bulb was too much...So I pulled it back 6 inches, now they are happy....
My opinion again - saying outright that the chicks will all die if you take the light away is a little drastic. Once they are fully feathered in, they dont need it. Last year in early march (night temps between 30-40) I had all of my month old chicks out in the barn, with out a light.
I think it mostly depends on the hardiness of the chicks, and the environment they are in...
Besides environment, best way to go is figure out when they are too hot or too cold...That illustration was a perfect example of what to look for.