If it's newly planted, and less than 6 feet tall, I'd say 5 gallons per week.
Adjust upward if its bigger or the weather is dry.
If it's more than a year old it needs no water.
It's a drought-tolerant tree, and I usually take that to mean, as Walter says, that it does not need supplemental watering after it is well-established.
Thanks everyone, but got my answer already.
I ♥ Lucy (couldn't resist)