The general rule is let them dry before watering again. I don't let mine go completely dry, only about 3/4. If the leaves start to feel soft to the touch, instead of nice and firm, or if they wilt at all, then you waited too long to water. You can check the dryness/wetness of the medium by leaving a bamboo skewer in the pot for a few minutes and pulling it out, or just judge by picking up the pot to see if it feels light. These are all things you have to figure out plant by plant. Hope this helps! TONS of information is in this thread, and it's worth the read:
http://www.orchidboard.com/community...ends-here.html