Young peach trees need 5 to 10 gallons of water each of the summer weeks, while mature trees only need 36 inches of water a year. Mulched trees will need less water than those that do not have a mulch barrier.
Once a peach tree starts to bear fruit, it will need more water than when it is not producing fruit. Deep watering done infrequently is better for peach trees than frequent shallow watering. Soil should stay moist for a depth of 4 to 6 inches near the tree. An easy way to check is to drag a finger every few days along the dirt.