The reason is that you probably got a mixture of grasses and rye comes up very quickly, tall fescue comes up a little slower and Kentucky Bluegrass takes 3 weeks to come up. KBG is the only one that spreads, so if you quit watering too soon, you'll have bare spots.

Once the three week mark has passed, water once a day for about 10 minutes. Then cut back to every other day for 15 minutes. You may need to water every other day for the rest of the summer.

Adjust your watering schedule for any rain that may fall. Next year, try to let the lawn tell you when to water. Too many people set their sprinklers to water every day all year long, so the most common lawn problems here are because of too much water (ironic, isn't it?). When the grass starts showing the first signs of stress (curling blades, doesn't bounce back when you walk on it), water deeply to encourage deep roots.

You should be able to get by watering once or twice a week once the grass has had a chance to establish good roots. If you have sandy soil, you may need to water three times a week in the hottest part of the summer. If you water deeply and infrequently, you don't need to worry about watering at night (all the lawncare books and magazines tell you not to do that, but they're written by people who don't live in the desert). If you water between midnight and sunrise, more of the water will soak in and less will blow away or evaporate.