BONSAI WATERING GUIDE
Second, assess your climate. Hotter, drier climates will require more frequent waterings, whereas cooler, more humid environments will require less. Again, you can offset this to some degree by the soil medium you choose for your bonsai trees - i.e. the heavier the organic matter used in a hot, dry climate, the less frequently you will need to water. Keep in mind, though, that the more rapidly a tree dries out and the more frequently you can quench the tree's thirst, the better the root growth and subsequent shoot growth.
Third, understand that your bonsai will have different watering requirements at different times of the year. In spring, as the new buds swell and foliage emerges, bonsai are sucking up a tremendous amount of water to produce these new leaves, so check your bonsai for water at least twice per day during this period. In the summer, as well, checking twice per day is a good idea. As temperatures drop in fall and winter, check once per day and water as the soil surface becomes dry (which could mean every day or once every 2-5 days depending on the tree).
How do you know when your bonsai tree is dry enough to water? Well, it depends on the individual tree and species; however, as a rule of thumb, water as the soil surface becomes visibly dry. If you are using Aoki blend or a mix of akadama, lava rock and pumice, this will be easy to determine, as the soil surface will appear light in color and will be dry to the touch. It is not necessary to stick your finger or a chopstick into the soil to determine how dry it is deeper within the pot. If the surface is dry, water thoroughly.
There are some species that require less water than others due to the potential to cause root rot. For example, Japanese White Pines usually require water only once per day, even in dry, hot climates. However, keep in mind that Japanese White Pine grafted on Japanese Black Pine root stock will need to be watered more frequently, as JBP roots prefer to remain slightly more moist than JWP roots.
The moral of the story, though, is to check your bonsai frequently and water as the surface becomes visibly dry. Keeping up with this approach will yield beautiful, quality growth on your bonsai trees.