Q.dormancy of christmas cactus
How long does the dormancy period last for Christmas cactus?

Certified GKH Gardening Expert
In order to encourage them to bloom, Christmas cactus should be placed into a period of dormancy by reducing watering, placing them in darkness for 14 hours each night, and placing them in a cool spot. This treatment should last for at least 6-8 weeks. See these articles:
https://www.gardeningknowhow.com/ornamental/cacti-succulents/christmas-cactus/how-to-make-a-christmas-cactus-bloom.htm
http://web.extension.illinois.edu/cfiv/homeowners/111122.html