Look at the grass, feel the soil, or use a soil moisture meter to determine when to water. Do not over-water. Soil can absorb only so much moisture, and the rest simply runs off. A timer will help, and either a kitchen timer or an alarm clock will do. One and a half inches of water applied once a week in the summer will keep most turf grasses alive and healthy.
Water lawns early in the morning during the hotter summer months.
Otherwise, much of the water used on the lawn can simply evaporate between the sprinkler and the grass. To avoid excessive evaporation, use a sprinkler that produces large drops of water, rather than a fine mist. Sprinklers that send droplets out on a low angle also help control evaporation. Set automatic sprinkler systems to provide thorough, but infrequent watering. Pressure regulating devices should be set to design specifications.