This question does not have a simple answer because irrigation requirements vary with grass species, with soil type, and with environmental conditions. These factors often interact in complex ways that make decision making difficult. On average, turf will usually require from one to one and one hald inches of water per week for normal maintenance conditions. This can be provided by rainfall or irrigation or a combination of the two.
The best time to irrigate is another management decision that can impact turf quality. Overly wet conditions in the canopy can contribute to disease development. Nighttime watering will keep the turf wet for the longest period of time and should be avoided if possible. Watering during the day will allow the turf to dry quickly, but will increase water loss due to evaporation. All things considered, the early morning hours provide the best time for turf irrigation.
Water loss from evaporation will be less, and the turf will dry quickly in the morning.