As with most plants, the only true answer to the question "how often do I water it?" is "when it needs it." It depends on the orchid species, the temperature and humidity where it is growing, and the light levels. Most epiphytic species--Phaelanopsis, Dendrobium, Cattleya, etc.--like the growing mix to dry out almost completely between waterings, like cactus. Terrestrial species--Cymbidium, Paphiopedalum, etc.--like only the top few cm of their growing mix to dry out between waterings.

Hope this helps!