The first two summers
Water the tree, not just the lawn. A young root ball can dry out even when the surrounding grass looks green.
Check before you water
Feel the soil a few inches down at the root ball. If it is dry, water slowly. If it is still wet, wait and check drainage.
Soak slowly at the roots
Use a slow flow so water soaks into the root ball instead of running across the surface. Check that moisture reaches below the top layer.
Adjust to the weather
Heat, wind, soil, tree size, and rainfall all change the schedule. The sample ten-gallon, twice-weekly rule is a starting illustration, not a prescription for every tree.
Keep watching as roots establish
Increase the interval as the tree establishes, checking the soil rather than relying only on a calendar. Established drought tolerance does not apply to newly planted stock.