If you want it to be truly dormant, 16 or 24 hours of light is fine, but the intensity of the light should be less than if you were trying ot grow or induce the plant to make cloneable shoots... do not overwater!!! This is the best way to kill a mother plant. The soil mix should be very very light and airy, lots of perlite, so she can dry out fairly well between waterings and not rot. And don't feed her too much... just a little every so often... VERY weak complete fert unless you are actually seeing deficiency signs.