Andy comments:

Set a can, like a Crisco can, in the yard and see how long it takes
to get an inch of water in it..... From then on, you can just leave
your
sprinklers on for that long,.,,

Bear in mind that different parts of the yard may get water at
different
rates..... You gotta use a little judgement, but the method is sound.

Andy in Eureka, Texas

PS It takes a LONG time for most sprinkler systems to deliver an inch
of water