Watering is necessary only when the grass starts to wilt. A dull darkening in the color of the lawn is the first sign it is starting to wilt. Footprint lying down after a walk across a lawn is another indication the grass has started to wilt. Immediate watering will quickly restore the lawn to good condition. Sandy soils will need to be watered more frequently then clay soils.
Slopes can be especially troublesome.
When runoff is a problem, the water should be shut off for half an hour, so the surface water can soak into the soil.