Your Bonsai should never completely dry out at any time. Generally speaking in the Summer you should be watering your Bonsai on a daily basis and on days of extreme heat it is not unusual to water them twice during those days.
During the cooler seasons watering can be less frequent and your Bonsai may only need watering every 2 to 3 days.
Be careful however during windy days as this type of environmental conditions can dry out your Bonsai very quickly even in the cooler months. So more regular watering should be done when windy days occur.
There are a number of ways you may choose to water your Bonsai and the way you choose will depend on your time available and the size of your Bonsai collection. One way of watering if you only have a few Bonsai is to fill a container up with water and submerge the Bonsai and wait for the bubbles to stop. Once the bubbles have stopped the potting mix in the container has reached saturation and you can pull the Bonsai out and that will do them for the day.
Another way is by hand watering with the hose as long as you have a watering attachment with a fine rose on the end of it so the water will come out softly on to your Bonsai. You may need to come back to the same plant a couple of times so the water definitely goes right down to the bottom of the pot and out the drainage hole.
The final way you can water is by setting up automatic sprinklers or drippers. This is a particularly effective method if you are time poor. These can be organised to come on a couple of times during the day. Be sure to still check your Bonsai particularly on windy days as sometimes the spray jets may be effected by heavy wind and not reach and be effective of their usual coverage.