How much
NC State: 0.5 to 1 inch per week, supplementing rainfall, watering to a depth of 4 to 6 inches. On sandy soils, up to half an inch every third day may be needed. Clemson: let the turf dry, then apply three-quarters to one inch, and let it dry again before the next cycle. UGA suggests about 1 inch a week in autumn when it does not rain.
How to know
NC State lists the signs of need: a dark bluish-gray colour, footprints that stay visible, and wilted or curled leaves. Water when you see them, not on a timer.
How to measure
Put straight-sided cans or a rain gauge on the lawn and time how long your sprinkler takes to fill them. One inch of water over one square foot is about 0.62 US gallons, so a 1,000 square foot lawn takes about 623 gallons for an inch. The lawn calculator does the sums.
Time of day
Early morning, so leaves dry quickly. Late-evening watering leaves turf wet overnight, which favours large patch and dollar spot.
Drought and dormancy
Zoysia that has browned in drought can recover if the crowns are alive. In winter dormancy, NC State says to water only if the soil is powder dry. Watering in dormancy is mostly wasted. See dormancy and winter.