Mid coast Maine??! Me too! Well, not quite mid. A little south of Mid, I guess but MAINE all the same.
My Buff Orps have their light on all the time. I suppose I should switch it to go off. Do you think it is better to have it go on early in the morning or stay on into the night? Maybe I'll try lighting the evening hours and we'll see who gets more eggs! We have had a LONG molt-most of August they didn't lay at all. I actually broke down and bought a dozen eggs. It was awful. Now they are back and I'm getting 1-2 eggs a day. (3 hens)
In answer to your question...I don't know! I'd guess a couple of weeks maybe.
Why not put a timer and photo cell over-ride in the coop. That way you would have a consistant number of hours light and no light if daylight made the electric light not needed? I have light and heat in my coop. Thermostat set at 35 degrees. I live in Michigan and get temps down to -20 degrees regularly. If you live in Maine so do you probably.