The grass blades that remain wet overnight are subject to invasion by all sorts of diseases, especially fungal diseases. Get in the habit of watering every seven to 10 days during the growing season, maybe every three to five days in the middle of summer. Depending on your local water pressure and the type of sprinkler you're using, you may have to water for an hour, even two, to really soak the soil. If you're trying to water a problem site, such as a slope or in soils that are heavily compacted, watch for runoff. To avoid wasting water, water the area a little bit until runoff begins, then stop, wait awhile, water again and repeat that process until the soil is thoroughly soaked.
Native plants work best.