Well, 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 water. You want to water deeply and infrequently to have a healthy deep rooted grass lawn.
Let's say you decide on 1 1/2 inches a week. You can do it in 2 doses or one, but don't do just a little each day. If you do a little each day your grass will have shallow roots and you can have 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 soon 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 it'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 not.
The best time to water is early in the morning. If you wait till afternoon you lose some of your water to evaporation. If you water at night you could be inviting diseases to your lawn because of the length of time it stays wet.
A couple tips to ensure your lawn looks great when it gets really hot out is to not mow as short and to leave the grass clippings on the yard.
By increasing your mowing height the lawn is less stressed and able to cool itself. Also grass clippings left on the lawn will actually give the grass's roots more moisture and minerals which will help it to be less stressed. Contrary to popular belief grass clippings will not cause thatch buildup.
So you see 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 when it comes to watering.