A general rule of thumb 1 inch of water per week is best during the seasons your lawn is actively growing. In the warmer months of summer you may want to increase this. When in the cooler seasons of spring and fall you might want to decrease your watering to 1/2 inch of water per week. In the winter months you want to water every few weeks depending on rain to ensure your lawn does not get too dry. Even when your lawn is dormant, roots are still active. While you don't want to water when it is below freezing you do still want to make sure your lawn is getting enough water that the soil is moist, to ensure your roots do not dry out.