When watering the tree, it is best to soak the soil well, then allow it to dry out somewhat before watering again. Of course, like most plants, you don't want the tree to get too dry! The rule of thumb for mature trees is about 20 gallons of water a day during the irrigation season. Seedling will require quite a bit less than that, of course.
At planting the trees can hold about 2 gallons of water in their rooted volume. Depending on the weather, they might use 1 gallon of water a day along the coast. Typically the trees need to be watered 2-3 times a week. As the roots reach out into the bulk soil, more water can be applied and frequency of watering diminishes to about 1 time per week by the end of a year. Check the soil before watering, to make sure it has dried somewhat. If the soil from around the roots can still hold the impression of the hand when squeezed, it still has enough water.