The two main types of watering are Establishment (used for new seed or sod lawns) and Maintenance (used for lawns already established).
The establishment watering is done by watering every day if not 3 times per day every day for short periods of time. First, water as often as possible, if you have an automatic system set it to water 3 times per day morning, early afternoon and 6 or 7 at night. Most of the automatic systems should be set to run from 5 to 15 minutes per zone depending on what kind of heads are running on the specific zone.
If watering by hand, water until the point of runoff then move the sprinkler. Use this method until the new grass is out of the ground and about 3 inches tall. At this time you are going to want to start cutting the grass. Turn the water off for a couple of days to let the ground dry out before mowing, then use a new or freshly sharpened blade with your mower set at 3” or higher. After mowing start watering again except back off by half, water every other day once a day for a longer period of time to start soaking deep. Slowly change your watering schedule to watering once or twice a week and watering for long periods on each zone to soak the ground to 6” deep. This will encourage deep root growth.
Also, do not water at night except during establishment. The first watering session to knock off is the night watering. Watering at night will encourage fungus to start on the grass and this is a very bad thing. Fungus will spread across the lawn in a week or less and kill the new grass as it grows. Fungus will look like straw colored spots in the lawn in a circular pattern. It looks like the grass is dying for lack of water. But more water just increases the fungus. Once a fungus gets started a Fungicide application is needed to kill it out.
Fungicides are usually labeled Danger, the strongest signal word on Pesticides. So it’s best to call a local lawn spray company to come handle it for you. Besides, the chemical is very expensive and sometimes only sold in gallon or larger containers that you would never use up.