It all depends. Christmas cactus thrive better when they are slightly root bound in their pot. However, you should consider re-potting when the roots grow too large for their pot (one tell-tale sign will be roots popping through the drainage holes of the pot). Christmas cactus typically only need to be re-potted every 3-4 years. The best time to re-pot a christmas cactus is after blooming ends and the flowers have wilted - it should never be attempted while the cactus is actively blooming.
If you do decide to pursue repotting, you can find more information here: