Lawns need at least 1" of water a week to keep the lawn alive and healthy. Once the frost is out of the ground, you need to water whenever there is no rain to accomplish the full inch of water needed. Get a measuring device from a garden centre (usually very cheap or free) and put it in the lawn somewhere where you won't step on it but it will catch the rain or spray from your sprinkler. Check it after a rain for how much water actually fell and after you water. I find it takes about 1 hour to get a 1" watering. Fertilizing in spring is also very important. You need a high nitrogen (the first number) fertilizer for the spring. Then fertilize at least 3 other times - early summer, late summer, and fall. The summer fertilizer can have the numbers equal for all 3 numbers on the fertilizer and the fall one needs the middle one higher to encourage a good root growth.