This depends mainly on the size and age of the tree. Regardless of how often a tree is watered, each water application should be enough to saturate an area as wide as the drip line of the tree to a soil depth of 2-3 feet minimum. As a tree matures, it needs a wider water area...especially for palm trees.
This depends on a few variables: Age of tree, type of tree, soil conditions and current weather conditions. Loose sandy soil dries out faster so more frequent watering is required where as tight soils may require less frequent applications. Newer trees don't have established root structures and require more frequent watering than when mature. Long periods of extreme heat dry out the soil faster dictating an increase in water frequency. During the extreme heat when temperatures are consistently above 110, add an extra day to your watering cycle.