Turf needs an inch of rain a week. 15 minutes hand watered is not enough. Usually with new turf or a new rose it gets watered up to 4 times a day depending on the amount of sun and wind it is getting. Depending on the kind of soil you have you need to water longer preferably with a sprinkler with over lapping coverage. You want to water until the ground is soaked at least 4 inches deep. The only way to test for that would be to actually test how deep the water went into the soil. I either dig a small hole after watering or I just put an inch of water on the lawn. You can test how long it takes a sprinkler to do that by setting out empty tuna cans every 2 ft from the sprinkler source out until the edge of where the sprinkler reaches. Sprinklers do not water evenly so the area they cover needs to be overlapped. When the middle cans fill with water you have put down about an inch. Cans on the outer edge of the sprinkler range are usually not full and neither are the cans closest to the sprinkler that is normal. How long to water will be the time it took for the cans to fill divided by the number of times you water in a week. For new grass I would err on the side of giving them more water so I would water new turf twice as long. For me that is the equivalent of 10 minutes for each section at 5 a.m, 10a.m. 2 p.m. and 5pm. It helps to either have an in ground sprinkler system or a sprinkler on a timer. If you have enough water pressure you can put up a temporary above ground sprinkler system.
Once the grass roots start growing into the soil, I will slowly reduce watering till I get down to the maintenance level of 30-45 minutes twice a week. What you have to do depends on the time of year and how well your soil holds on to water.
My water pressure is poor so it takes longer to put down an inch of water. If you have better water pressure it won't take as long.
How did you prepare the ground and did you get a good seal with the sod and the soil?
Happy gardening in Hawaii. Gardens are where people grow.