The main thing to remember about watering established Orchid Cacti is that the mix should never be allowed to dry out completely. Use your finger to determine when this has happened. Nothing replaces checking the mix every few days to see whether it is time to water. Generally speaking, clay pots will dry out faster than plastic ones. Plastic potted Epiphyllum require less attention then ones in clay pots.
Orchid cactus go through natural growth cycles in the spring and again in the fall.
Naturally, they demand more water during this time. The plants usually rest after flowering and again during the winter months. Even if your plants look a little wilted from the stress of producing flowers, do not increase the amount of water that you have been giving them. This is a natureal phenomenon and the plants will usually recover their plump look by fall. During the winter months, give the plants just enough water to stay moist.