Water OD, pretty much the same as ID. "When they need it"
Environmental conditions, the ability of the ground to retain or drain, plant size, are all factors related to how frequent you'll need to visit. There is just no way to tell you "How often or How much" exactly.
In "my world", twice to three times per week, depending on rainfall, ect.
Unfortunatly, the effects of both over and under watering look similar, it takes experience to tell them apart by plant appearance.
Stick your finger all the way into the soil 6" or so from the plant. If the tip of your finger is wet/moist, hold off on watering.