How often should I water a Christmas cactus plant in a medium size pot

Water when the soil is bone dry about half the depth of your finger.

Don't water a Christmas cactus (Schlumbergera x buckleyi) until it tells you it needs it. Like many succulents with thick fleshy leaves, the leaves will just start o shrivel a bit when they're thirsty. Over watering is the absolute best way to kill this plant.

Here's a bit of info: http://houseplantcentral.com/schlumbergera-christmas-cactus-care/

Get a moisture meter that will help to tell you when the soil is dry enough to water. It may save your plant from over watering.

I water mine once a month in the hot months, and once every two or three months in the cooler months. If I forget to water one time, I make myself wait until the next month. Then I check the soil down about two inches. I have killed cacti of all kinds by watering too much. ☺️