How dark does the dark cycle need to be?

Consider that Cannabis can flower outdoors under the light of the full moon, so if your grow room is that dark, you should be OK. Having said that, it's best to make it as dark as possible. Plug all light leaks and be liberal with the black plastic.

Be sure to let your eyes adjust to the darkened room for at least two minutes before you try and identify how "truly dark" it is. If you can hold your hand out at arm's length and see your fingers, it is probably too bright. Light leaks are common triggers for hermaphroditism.