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 of water 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.