Determining how long to water new grass depends on several variables. The shortest answer is “Never let the new grass dry out.” It should stay damp, but not saturated for several weeks.
Please be sure to have enough hose, tees, timers, and sprinklers to cover all portions of the new turf on site prior to the start of the project. Please make sure the hose bibs are turned on, de-winterized, and functioning properly.
Watering new sod properly the first day is super critical because proper watering can keep all the existing blades of grass alive. Insufficient watering will force the sod to kill off some of the foliage to protect the rest. It will only grow new blades when there is a sufficient root system to support the extra demand. The difference is a density issue. Regaining the super thick grass the farm supplied after a kill off is increasingly difficult the more time has passed. Be prepared to water daily the first week 20-30 min in each sprinkler location and some times twice a day if extra high temperatures and/or drought conditions exist.
The sod will need watering within the first few hours, not days. It has approximately 1/10th the root system a mature and healthy lawn should have. The first growing season is by far the easiest time to help the turf establish an optimum root system. The soils are looser and allow vigorous root growth. The young sod tends to grow more quickly, immediately following installation because it has energy reserves created at the sod farm where the growing conditions were ideal. I usually recommend watering every day for the 1st week, every other day the 2nd week, every third the 3rd week etc. The idea is to slowly wean the grass off of artificial watering. This doesn’t mean ignore the grass during hot, dry times. Our unpredictable weather patterns will require supplemental watering. Generally when temperatures rise above 90, or your lawn goes without rain for over a week, it is time to water.
Automatic irrigation systems are now available with all sorts of sensors for, rain, humidity, in ground moisture, and temperature, to automatically adjust time and frequency of watering & create optimum growing conditions. I highly recommend one of these systems because they are more efficient while still providing the idea level of irrigation.