What you want to do is cut back the frequency not the amount of time your sprinklers run. In many cases once a week watering is enough. If you cut back the amount of time you won't be watering the entire root zone.

Mistake!

Always water long enough to get the water into the soil and down to the roots.

If you have run off problems, water 2 or even three times in succession at 1 hour intervals that allows the water to penetrant the soil the way you need.

I am watering my lawn twice a week and am about to cut back to once a week. My shrubs and trees are being watered once a week and about to be cut back to every two weeks.

---