How long does a Christmas cactus need to be kept in the dark?

https://www.gardeningknowhow.com/ornamental/cacti-succulents/christmas-cactus/how-to-make-a-christmas-cactus-bloom.htm

This link will give you all the help you need.

https://www.wikihow.com/Care-for-a-Christmas-Cactus

II have an 85 year old Christmas cactus. I've had it for at least 25 years. It is always kept in an area with indirect high light. Every spring I put it on the deck where it gets afternoon Sun. Only the rain waters it. As soon as nights drop below 50 I take it in. It blooms faithfully every Christmas and around June.I have to say that I often feel that I neglect it more than anything.

They will bloom regardless but the florists keep them in a dark cool room for 12 hours and then bright daylight for 12 hours just prior to the holiday season, like November. Same with poinsettias.