Water to a depth of six inches. This will probably be necessary every 4 to 7 days, but may vary with the time it takes your lawn to drain. Avoid evening or late night watering as this has the potential to result in conditions for disease or mold. 
 

Do not water on a predetermined schedule, but only as needed. When the first signs of water stress are detectable. When water stress occurs the grass begins to turn slightly blue and footprints persist when you walk on the lawn. Another term for water stress is wilting.
 

If water is in short supply and the grass is allowed to go dormant, do not water to restore growth unless the water can continue in adequate amounts. Once new growth occurs it will suffer serious injury if allowed to dry out again.

Note:  You can measure the amount of water delivered by a sprinkler by setting out an open can within the sprinkler pattern. Measure the depth of the water collected in the can. When the water reaches a depth of 1" the lawn has had enough water for that duration.