Three days after planting: fill the watering basin three times using a total of 15-20 gallons of water. This initial watering is very important as roots are the most sensitive right after planting.
For the next 3 weeks: fill the watering basin once a week with 5-10 gallons of water.
For the next 6 months: following the planting, fill the watering basin every week or every other week with 10-15 gallons of water.
For the remainder of the first year: water every other week with 10-15 gallons in absence of soaking rain. Roots need oxygen just as much as they need water, and conservation at this stage will actually help your tree grow!
When rain is scarce, usually beginning in late spring, begin watering again every two to four weeks with 15-20 gallons of water, depending on the tree species.
Allow roots to seek out moisture deeper underground and further from the trunk, creating a resilient tree. Shallow watering leads to shallow roots and water dependency. Deeper, infrequent watering will lead to resilient trees with less impactful roots.
Maintain the watering basin so it continues to hold water, and spread it out to hold more water over time.
Third to Fifth Year:
For years 3 to 5, water at least once a month with 20-30 gallons of water. Monthly soakings should maximize growth while conserving water. Many irrigation timers actually have a setting for monthly watering. You can also set several start times 15 to 30 minutes apart, to decrease runoff and improve soil penetration.
Over the next two summers experiment with decreasing frequency and increasing duration and coverage.
Be wary of prolonged dry periods, proper summer watering can mean the difference between a tree that thrives and one that is permanently stunted.
Between 3 and 5 years old, your tree should be fully established. Occasional deep watering, especially during times of drought, will help your tree thrive.
Source: Caring for Young Trees