It is stressful to the hens to have lights on 24/7, they really do need some dark time for sleeping. I don't use lights so I can't comment on how many hours to leave them on, I believe it's 14 or 15 hours of light a day but don't quote me on that!
I emailed Dr Bramwell, one of the top poultry reproduction specialists in the country, about if it Iis possible for a hen to run out of ova. Here is his response. It’s possible but unlikely.
Yes, a young Pullet chick hatches with a determinant number of follicles on her left ovary that are possible to be developed to become one of the yolk or ova in an egg. Only the left ovary develops in most avian species so anything on the right ovary never develops and is of no use. So can a hen theoretically 'run out of' potential follicles? Yes she can and this does occur on occasion. It happens often in parrots that may live 70, 80 or even up to 100 years of age. These hens that run out of follicles are referred to as 'slick hens' as the ovaries are often checked by veterinarians and confirmed that they are indeed 'slick' hens with 'slick ovaries' that have no more follicles to develop.
So does this happen often in chickens? Not really that often because most of our domestic chickens do not live long enough to run out of follicles, they 'run out of gas' in other ways much sooner. This could be some other disease or infection or condition related to extended periods of laying that might either cause death or an inability to produce and develop proper shells on an egg.
So in short, yes it's possible but unlikely for a chicken to become 'slick' and run out of follicles.
Dept of Poultry Science
University of Arkansas
I highly recommend you follow that link in Aart’s post. It’s full of good information. One important thing, it’s not a magic number of daylight or dark hours. It’s nights getting longer. Unless your chickens are kept out of the sun completely you need to time that to your longest days or pretty close to it. Chickens near the equator never see 14 hours of daylight yet they go through the same cycles.
I agree with Cafarmgirl it is very stressful to have lights on continuously. If you check the Egg Quality Handbook, several egg defects can be caused by improper lighting. This improper lighting is often leaving the lights on continuously.
That's what my research has led me to believe. That chickens slow down laying because of increasing age and declining condition. Not because you "wore them out" with supplemental winter lighting. Yet people will cling to this belief because it suits their intuition.
Furthermore I doubt that giving hens "time off to rest" in the winter is of any benefit to them. But I have yet to see any authoritative information on this. Perhaps only the hens themselves have the answer.
There is literature out there that shows hens need some time off. Aart’s article mentions it. It’s not tied to an annual schedule though that makes it easier for some of us.
Commercial operations track egg quantity and quality. After a certain time they either molt the flock or replace them because the profitability goes away after them laying a certain length of continuous time. That affects both numbers and quality of eggs. You can find productivity curves that show when a flock needs to molt or be replaced depending on tracked productivity.
I think breed or at least heredity has something to do with it too but that’s my opinion. I don’t have any research to back that up. Practically all the research I’ve seen has been on the commercial hybrids not our backyard chickens.