Place a small tuna can or some other inch-deep container in the middle of your lawn and turn on the sprinkler. When it's full, you're done. Note how long this takes and use that time for future waterings during similar weather.
If you have slow-draining clay soil, you may need to split your inch of water into two waterings with at least an hour or so in between to let it soak in. Or you could water 1/2 inch twice a week. If you use a hose-end sprinkler, battery-operated, programmable timers are a great way to deliver just the right amount.
When it comes to water, more is not better. Overwatering can stress a lawn as much as drought. When it rains, water less, if at all. If you have an irrigation system, adding a rain sensor to your timer will save water and protect your lawn.
Find more watering tips from the Regional Water Providers Consortium