As a general rule, you should water long enough to moisten the soil down to about 6 inches, which is the average depth of a healthy grass-root system. It takes about one inch of water to properly hydrate normal lawns to that depth. You have to water for your specific property.”There are a few ways to determine whether your lawn is getting the right amount of water. After watering the lawn, take a long-blade screwdriver and shove it straight down into the lawn. Multiply the square footage of your lawn by 0.62 gallons; that’s the amount needed to reach one inch of water per square foot.