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.