Providing supplemental water for a tree purchased from us is the most important component to that trees success. The general rule of thumb we provide is to water the trees weekly for the first month with the same volume as the pot. If the tree came in a 2 gallon pot, it should get two gallons a week. After the first month, water once every two weeks, with twice the volume of the pot.
Beginning In September, decrease your watering schedule. This encourages the tree to 'harden off' and start getting ready for winter. When the leaves start to turn color you can give them a good deep soak. (Combine with your fall fertilizing) This allows the roots to pick up a bunch of last minute nutrients, and start getting ready for spring.
These recommendations are for clay to clay loam 'heavy' soils. If your site is sandier, I would double the recommended watering volume and schedule.