Light sprinklings do more harm than good on an established lawn, so water deeply.  If you want your lawn to remain deep-rooted and healthy, water until the soil is wet to a minimum depth of six inches.  Twelve is better yet.

Once watered, lawns hold their moisture for a remarkably long time.  More lawns suffer from excessive watering than from drying out.  There are several signals when grasses are about to dry out: 1) turf loses resiliency so you can see footprints or mower tracks long after you've crossed the lawn; 2) grass assumes a bluish cast; and 3) growth is substantially slowed.  When these warnings appear, it's time to water.

A lawn with fairly heavy soil will require around three hours of watering for moisture to penetrate to the proper depth.  But sandy soils don't hold moisture well, so lengthy waterings are of little value.