Depends on where you are located. It should be at least 1 inch of water twice a week. Anyway thats the rule of thumb for here in Florida.
If you are not sure how much water your lawn is getting, get several empty tuna cans or something of the same size and place them around your yard. When you have one inch of water in every can you will know how long to water. The early morning is best as stated in previous posts. To water in the evening may leave too much moisture overnight and invite mold diseases. Never water when the lawn or plants are under direct sunlight.
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