Irrigation should start when soil moisture levels drop to a critical level and end when the soil is recharged. In practice, most permanent irrigation systems are timer based without soil moisture sensors. These systems may need to have the timer adjusted several times during the growing season in order to provide adequate amounts of water or to avoid over-watering.
This type of system should be set up based on plant type zones. Turf is usually irrigated lightly and frequently while trees and shrubs are irrigated less frequently and more deeply. As such, it is important to have an arborist inspect your trees to advise the appropriate action.