Same time you'd discontinue a lamp.When do you discontinue the heat plate? I'm starting my chicks in the spare bedroom because the weather has been a bit nasty. In the teens at night and maybe up to 35 or 40 in the daytime, But an extremely cold NE wind howling thru. Hopefully in a few weeks the temps will raise a bit.
I've made a brooder pen for them with a dog x-pen, and thought I'd move the whole set up out to the coop in a few weeks.
Does your coop have power?
Is it blocked from wind inside?
Lots of variables to consider.