There is no set formula for how often to water a bonsai. Just keep the soil slightly moist at all times. Never let it dry out completely. Here's the right way to do it:
Water thoroughly with a gentle spray from a sprinkling can, syringe, or hose whenever the soil starts to get dry. Get in the habit of checking the soil daily for dryness. If soil feels moist, don't water. If it feels dry, water thoroughly.
During very hot weather, place your bonsai rim-deep in a sink or pan of water for 5 to 10 minutes once a week. Do this in addition to normal watering only to ensure that bottom roots get their share of water when it's hot.
Soil dries out not only because the tree takes up water, but wind, heat, dry air, and strong sunshine dry the soil, too. More water is needed in spring and summer than in cold weather. In extreme cases of hot, sunny weather, you may need to heavily water two or three times a day.
A helpful hint is to keep the tree in a shady location during these times may reduce the need for such frequent watering. During cold spring and autumn weather, light watering may be necessary only once every several days. Generally, too much water is better than too little, provided the tree is getting plenty of sunshine.
During warm or hot weather, misting the leaves or needles and moss (in addition to normal watering of the soil) is beneficial. Here's how to do it: Using a misting device (such as a empty, and clean Windex bottle). Spray the foliage and moss, daily if possible.
The Japanese know how to keep their miniature trees alive and healthy for many years. They do it by having a "growing place" and a "showing place." Normally, the growing place is outdoors in fresh air and sunshine, usually on a table or shelf on the veranda or on pedestal in the garden. When guests come to visit, a favorite bonsai is brought indoors and placed in a spot reserved for special displays,- this is the showing place. When guest depart, the tree is returned to its growing place outside. Those fortunate enough to have a collection of bonsai can have a tree indoors all the time - by simply rotating them daily.
Americans, referred to as westerners, by the Japanese, are discovering this technique which results in long-living specimens. Using this method, a bonsai can spend most of its time in a natural outdoor environment, enjoying the changing seasons - and also present an attractive display in a reception room, office, or living room.
What ever you do, please remember that bonsai are not house plants and they need a balanced diet of fresh air, natural light, and water to remain healthy. And yes a bench placed under a shade tree, watered by an automatic sprinkler system, is an excellent location for your bonsai.