12hrs (some debate on this)complete dark in flower.
No debate about that...12 hours of uninterrupted darkness is a MUST. Otherwise, you WILL get a hermie. Also, if you are not sure about light leaks, then when your lights go out go into your grow room, close the door, and look for any light leaks and be sure to seal them. You could lay down a small towel or cloth at the foot of the door to seal that leak.

Also, I have a little heat problem as well (85-90) which is why I have yet to start flowering mine. If the heat is too high during the flowering phase they will beome airy and not dense buds.