The very best time to water your lawn is the early morning, before 10AM
Cooler temperatures and calm breezes help keep evaporation to a minimum and keeps the turf cooler during the hottest parts of the day, which means less stress on the grass.
If it’s not convenient to water in the morning, late afternoon is the next best time, but don’t wait too long. Although it might seem smart to wait until night, when temperatures are cooler, watering in the evening keeps lawns wet overnight, which can make the grass susceptible to disease.
As a general rule, you should water long enough to moisten the soil down to about 150mm, which is the average depth of a healthy grass-root system.
It takes about 30mm of water to properly hydrate normal lawns to that depth.
Of course, every lawn has different soil so you have to water for your specific property.
The simplest way to determine if the soil is properly watered is to conduct the screwdriver test:
After watering the lawn, take a long-blade screwdriver and shove it straight down into the lawn. The blade should easily penetrate the soil to a depth of about 150mm.
If it doesn't, the soil is hard and dry and you're not watering enough.