They can sleep for as many or as little hours you want them to. Marijuana is not very needy. I keep mine on 20/4, 4 hours of darkness. Alot of people do 24/7 lights, do whatever you please. If you're transplanting outside though you may want to start decreasing the amount of light they get down to 18 hours at most so they don't flower on you, you can do this a week before the transplant.