I have read several articles on the night blooming cereus and none tell how much and when to water?. Also, mine is quite large and when should it be re-potted?
Night Blooming Cereus
Certified GKH Gardening Expert
These will only need water when the soil is almost completely dry on top. This can make it impossible to give a definite time frame to water these, as this will depend on the temperature, humidity, and many other factors. So the best way to check is just to stick your finger in about an inch or two, and if it is dry down to that, then you can water again.
As far as when to repot... You will have to lift the plant out of container, and if the roots are circling the bottom, it will be time to transplant.