You water when your grass shows signs of needing water.

Depending on your soil, that could be tomorrow or 2 weeks.

Watch the grass for signs of drought stress. The leaf blades will start rolling up and might look more like needles. Areas in the lawn will start to take on a silver or gray cast when you look out across the lawn. This may occur first around trees, or on top of hills in your lawn.

When you see these signs, it's time to water.

I disagree....you should not wait until the grass shows a problem; you should always try to avoid the problem by giving the lawn what it needs....now....not tomorrow.

You cant wait two weeks, then say you'll give it two inches of water. Using that reaosning, then let's wait 6 weeks...and give it 6". Of course, that reasoning will just end up with a dead lawn.

Give your lawn...as the experts agree...1" per week....
every week, either by rain or hose.
When a lawn is let to dry out, it encourages shallow rooting. The thinking then is if given water, the roots should grow again. but why not avoid the shallow rooting...give it moisture as it can use it to keep strong roots.
Giving a lawn that extra amount will also cause problems.
Too much water causes exactly the same result if given too little.....short roots. Grass roots can drown---just like when we drown a houseplant; the roots stop taking in moisture...they die and the plant above is starved.

"I disagree....you should not wait until the grass shows a problem; you should always try to avoid the problem by giving the lawn what it needs....now....not tomorrow. "

I don't disagree with that. That would make better sense.

Learn the signs of drought stress. Learn when your lawn *usually* shows drought stress after a given amount of irrigation, and water in a manner to keep that from happening.

I guess there really is no sense in letting your lawn get stressed if you know when it likes to have a drink based on its history.