How long should I leave a Poinsettia in a complete dark room in October?
It is best to put the Poinsettia in the dark from about 5-6pm until 5-6am every day in October and November. You can pick which 12 hour slot works best for you but give the plant less than 12 hours of light. You can just put a box over the plant rather than move it every day.
Then starting at the end of November give the plant a sunny window with a full day of light. Since we have short day lengths that time of year you may want to supplement the plant with extra light for a few hours.