New plantings need more water than established plants. For the first week water about 1" everyday. Each individual plant should be watered at ground level of the drip line. For the rest of that growing season water perennials 2-3 times a week.
Established perennials need about 1" of water a week. Deep watering once a week encourages deep rooting, as apposed to daily quick watering which keeps roots shallow.
Watering should be done in the cool weather of the morning. Watering during the middle of the day can cause more water to evaporate and is less efficient. Watering at night can cause mold and mildew, as the watering area will stay damp all night.
Try to avoid watering the foliage. While a sprinkler is very convenient for watering lawn areas and large areas the most efficient way to get the water right to the roots is with a soaker hose or holding a hose at the drip line for a few moments per plant.