Water wicks up the tower via evapotranspiration, so the best place to test the soil for dryness with your finger is a pocket in the lowest row.  If the soil feels dry, a heavier watering is required (5-9 gallons), if the soil feels slightly moist a maintenance watering is all that is required (2-4 gallons).
If the plants look droopy or dry you need to water. But generally, it depends on the weather, the types of plants you are growing, and your soil mixture. We cannot tell you when to water. Like watering your house plants or garden, you'll get a feel for it. The drain at the bottom eliminates the possibility of over-watering.