Contrary to popular belief, it is best to water your lawn deeply and occasionally as opposed to frequently and lightly.
Lawns generally need at least 1 inch of water per week during the warmer seasons. If in doubt, use a rain gauge to measure. Water once or twice a week but deeply to reach the roots. Frequent light watering wets the lawns surface and forces the roots to stay shallow. Deep watering allows the roots to chase after the water and promotes deeper rooting and helps lawns survive stressful periods of heat and drought. Consider watering more frequently during the heat of the summer and less during the cooler spring and fall. Little if no watering is required in the winter.