I do it when my sprinkler company tells me they're going to do it!

I have read that a sprinkler should provide your lawn with 1 inch of water per week, preferably all in one watering. Turn on your sprinkler and set an empty tuna can under the spray so you can see how long it takes to get 1 inch of water in the can.

If it takes 4 hours to get 1 inch of water in the can, water once a week for 4 hours.

Try to avoid watering mid-day, as this is when it is the hottest. Morning watering from 6-9AM can often be best, for about 20 minutes (but will vary depending on your system). Do this about 2 times per week. It is also a good idea to install a rain sensor or moisture sensor to prevent watering unnecessarily after a storm.