You'll know I have two and they started screaming becuase they were cold. I wouldn't turn of the light if the room temp is below 75. I had to move these guys back into the office where it's a comfortable 80.
The "age" that they no longer need controlled temperatures of any sort, such as that provided by a light, is "fully feathered".
That seems like it falls anywhere between 6 and 8 weeks, but as with anything else when dealing with animals it varies from individual to individual.
You'll have to see what your chicks tell you. I'd turn your light off during the day -- for a long enough time for the temperature effects to be felt -- when you can observe them and see if they behave as though they're cold. If they don't (and you're sure your house stays a consistent temp; mine doesn't), go ahead and turn it off at night. If they do behave as though they're cold, try it again in a week.
Be aware that they will cluster a bit the first time they're in the dark. That doesn't necessarily mean they're cold; it means they're scared.