Once your lawn is established the amount of watering that it needs will be generally dictated by the weather conditions.
The method used for watering is not critically important, but some methods are more efficient than others. Hand watering by watering-can or hosepipe is considered by most to be less beneficial and efficient for the lawn than a sprinkler system. Good watering is not how long you water for but ensuring that it the grass roots themselves receive water.
Try not to over water the lawn but at the same time give the roots water they need. One relatively easy method of determining if the roots are moist is to push a knife into the soil. If the knife goes in easily and when you pull it out it is cool and clean it is likely the soil is damp. If the knife won’t penetrate easily then you probably need to water.
Your lawn, in general, will require water when it is growing and thus during the times when you are most frequently cutting the grass. Remember to water the lawn early in the day, ideally about 10-00hrs so it will be dry by the evening. As a rough guide from June to September your lawn will probably need to be watered about once a week. During the spring and autumn, the turf will generally require much less water than the summer unless there is an exceptionally hot dry or windy period, and during the winter the turf does not need to be watered at all.
If, however, at any time there are signs of the turf drying out it should be watered immediately.