All living things need water, your lawn needs at least 1"-1 ½ " of water per week, year-round, during the winter, too. Lawns that are dry going into winter or during winter are more stressed and more likely to sustain winter damage. How do you know how much you're getting? First of all, buy a rain gauge. If you get an inch of water in it per week, you're doing OK. If you don't, you need to water.
How long should you water to get one inch? Usually 20 minutes, 3 times per week will get an inch of water on your lawn, and 30 minutes 3 times per week will get 1 ½" down. Bear in mind that during extreme heat, you will need to water more, due to evaporation and heat stress on the grass.
Water deeply 2-3 times per week, rather than daily. Watering daily will give your grass a shallow root system. Shallow root systems dry out fast and weaken your turf. Watering deeply 3 times per week will give your grass a deep root system, making it stronger and more drought-resistant.
Water as early in the morning as you can, when possible. Watering early in the morning will ensure that your lawn dries completely before nightfall. A wet lawn at night, on a regular basis, can lead to fungus and disease problems. Also, it's cooler and less windy in the morning, so you have less evaporation, which will save money on your water bill and take less time to get the water on your lawn. If morning watering is impossible, watering at any time is always better than not watering at all!