After the first 2 years, you never need to water them. They tap into the ground for their water. And during those first 2 years, the water they need is about 10 gallons per week, during the dry season. When we wash & prep the parsley we eat in one week, we create 2 gallons of waste water (that could go to that tree). Clothes washers create gallons & gallons of gray water that can be used to water trees -- in our household, we produce 150+ gallons of gray water from clothes washing alone.
It takes ridiculously small amounts of work and creativity to have enough water for outdoor plants without ever having to turn on a tap.