While your soil type, the amount of sunshine, and the climate all come into play, a good rule of thumb is 1 to 1 1/2 inches a week.
What is most important is how you actually water your lawn. You’ll want to water deeply and somewhat infrequently to have a healthy deep rooted grass lawn.
Let's say you’ve settled on on 1 1/2 inches a week. You can do it in 2 watering sessions or one, but don't do just a little bit each day. If you do a little each day your grass will have shallow roots and you can end up with all kinds of drought problems.
Now, if you have sandy soil, it is best to water your lawn twice a week since sandy soil drains faster and before long, the water becomes too far down for your thirsty grass roots to reach it.
By the same token, if you have dry, heavy, clay soil you may need to water 3 times a week due to the soil’s water repelling qualities.
If you are unsure how much water your sprinklers put out in an hour, then put out a few plastic tubs to measure how much water your lawn is getting and then adjust your watering time accordingly. Also, by putting out several tubs you can see if your sprinklers are watering the whole yard evenly or wasting water somewhere.
The best time to water is generally early in the morning. If you wait until afternoon, you lose some of your water to evaporation. If you water at night you could be inadvertently inviting diseases to your lawn because of the length of time it stays wet.
A couple tips to ensure your lawn looks great, even when it gets really hot out, is to not mow it short, increasing the stress on the grass.
By increasing your mowing height, the lawn is less stressed and able to cool itself. Also some grass clippings left behind on the lawn will actually give the grass’ roots more moisture and minerals which will help it to be less stressed.
It is just as important how you water as how much you water. It is not that difficult to keep your lawn green and lush with a little planning and thought when watering.