This doesn't seem complicated, but there's definitely a wrong way to water your lawn. Many people assume that a little watering every day is the way to go, but you can actually hurt your lawn by watering too frequently and not enough at any one time.
Assuming you're growing grasses and plants native to your area, a good rule of thumb is to emulate nature. In much of the eastern half of the United States, a good rainfall might occur once a week on average and last an hour or more. This gives plants a good soaking that goes down to the roots.
When you only water for 20 minutes, all you're doing is soaking the surface while the deeper roots remain dry. If you do this too frequently you will also encourage the growth of moss and water-borne diseases.
Experts say the optimum amount is one inch per watering session. You can measure this easily by placing an empty tuna can or other small container in the yard and keep the water on until an inch of water is in the container.