For almost all plants, a deep, thorough soaking followed by enough time for the soil to dry out slightly is ideal. Frequent light watering is not good for plants. It encourages shallow root growth. That's why irrigation systems designed for the lawn are seldom adequate for landscape plants and if they aren't set up and operated properly, they can actually be harmful to a lawn too. Whenever possible, plants should be watered early in the day for several reasons.
Plants use the most water during the warmest part of the day.
When you water early in the day, you avoid losing as much of the water directly to evaporation. Also, watering early in the day allows the plant foliage time to dry out before evening, minimizing problems with fungal diseases. The amount of water that falls in a given area can be measured by setting out several shallow, straight-sided containers such as tuna or cat food cans. Whether there is a water shortage or not, it is always a good idea to take a few steps to conserve or minimize your need for water. It also keeps the surface loose and cool and slows down water loss due to run off.