I used to leave a 40 watt bulb on 24/7 through the coldest weeks of Winter. The birds layed well and they could eat at anytime they wanted, day or night. I thought that was important.
When it was warmer, I went back to keeping the light on about 14 hours each day.
This year, I'm allowing them their hours of darkness right thru the Winter. Obviously, I wasn't adding any heat to the coop with only 40 watts. In fact, the light that I'm using now only uses 11 watts (compact fluorescent).
I have read that there is no benefit for greater than 17 hours of light. Lighting is important to egg production and it is an important expense for the commercial outfits, as well. The poultry scientists have studied the lighting issue exhaustively.
24 hours of light - - Of course, the hen's laying cycle would be not the same as with natural daylight. A concern is behavior problems (pecking). This is more of a light intensity issue, however. Bright lights cause problems but for commercial purposes, the birds will not get any more light than is necessary.
For the backyard chicken-keeper, I think the idea if you are using lights for heat is to get them in the infra-red part of the spectrum. Pumping the heat equivalent to a 1,200 watt bathroom heater into a coop as broad-spectrum light would be almost too bright to imagine in most of our small backyard coops.
Steve