The only way to know for sure how much your lawn is getting when you turn on your sprinkler system is to measure. First, make sure all sections of your lawn are getting coverage from your sprinkler system. If not, make adjustments.
Next, set out small containers (tuna cans, water bottles or soda cans cut in half, etc.) throughout your lawn, let your irrigation run for 20 minutes and measure how much water collects in each.
With this data, you can determine if you need to add or adjust sprinkler heads as well as how long you need to water to dispense 1 inch of water.
Water deeply and less frequently. Water in the early morning to make sure less water evaporates in the heat of the day.
If you determine that you can deliver 1 inch of water to your lawn by watering it twice a week for 20 minutes at 6 am, you will want to have a rain gauge to measure how much your yard receives whenever it rains.
That way, you can turn off your irrigation system when rain is forecasted and turn it back only when needed.
By adding organic material to your lawn (topdressing with compost or well-rotted cow manure), you can improve the water holding capacity of the soil beneath your lawn while also providing much-needed plan nutrients.
This practice should not only give you a healthier lawn but also enable you to cut back on watering and save you money.