Water generously, with your hose on gentle spray, or use a watering can with a rose attachment. Keep watering until the water runs out the bottom of the bag. How often you should water will depend on the kind of potting soil you use, and how dry and hot the air is. If it's windy, your plants will also dry out more quickly.
As with almost any container garden, you’ll need to check the soil at least once a day, to see how moist it is. To check, stick your finger up to the second knuckle. If the soil is dry at your fingertip, water your plants until the water begins to run out of the bottom of the bag. It is better to give plants a really good soaking less often, than to give them just a sip.