It takes about two years for the roots of your newly planted tree to become established and grow beyond the planting hole. Routine watering is critical during this time. Root growth is slow if soil around the root ball is too wet or too dry so, in the first two years, you want make sure the soil in the root zone is evenly moist.
After you plant your tree create a water basin. Dig a donut shaped ring an inch or two away from the trunk and wide enough that it encompasses the root ball and slightly beyond (the root zone). Each time you water, fill the donut ring to the top. This ensures water will seep into the soil around the root ball rather than flow elsewhere.