I turn my heat lamp on whenever it drops below freezing at night. I only have hens from the original chicks I purchased several years ago and haven't added any since. They are getting older and in an effort to make sure they are as comfortable as possible from certain signs of aging, like arthritis, I think the benefits of the extra heat are more important.
Also, a thought about the adaptation of chickens to cold weather: Artificial selection has dramatically decreased chickens' abilities to survive on their own. Our backyard hens are not the same jungle fowl from southeast Asia. Some have feathers on their feet, on their head (Polish, etc), and extravagant combs. Even our "hardy" breeds like Wyandottes, Rocks, and others, were selected ultimately for egg and meat production. While they will fare better than a Sebright, they still are domesticated! The entire process of domestication for any species reduces body size, strength, and hardiness.
I don't understand the argument that "a little frostbite is ok" because it hurts like heck! And even if frostbite doesn't occur, imagine spending an entire night with your fingers frozen. Chickens can tolerate lower temperatures than we can, but to figure out where that line is, the best thing is to look at your birds' behavior because they're individuals and no formula will work reliably. Use their breed and age as guides. If they puff their feathers up more than usual or are crowded together or under a heat lamp, they need more warmth.