You water when the medium you are growing in is getting dry. The roots need oxygen so you shouldn't keep it really wet. You just want it to be damp so you can either lift the pots and go by weight or stick your finger into the soil. After a while you'll just get a feeling for when your plants need water but until then use one of those methods. Remember most people over water in the beginning until they know what they are doing so really watch out for that.