The best thing you can do for your newly planted tree is to give it the correct amount of water on a consistent basis. This should be done in a cycle so that the roots and soil have time to dry out between applications. This will establish a strong root system, rather than applying the same amount of water every day.
Generally, trees should be watered about 2-3 times a week during the growing season. However, this really depends on weather conditions such as rainfall and temperature. Sometimes a tree may need more or less than this recommendation.
To test the soil push your fingers a couple of inches into the dirt beside the root ball of the tree. If the dirt is moist 2-3 inches down, then you are in good shape; if it is dry or you have a hard time pushing your fingers into the dirt you need to add water. How long you need to leave your irrigation system on to achieve this soil moisture will depend on how well your soil retains water so there is no hard-and-fast rule. Once you test the soil a few times setting the run time on your irrigation system will become clear.
Usually the watering time is about 7-12 minutes to accomplish this. Soil composition will vary with location so it is important to monitor the areas where you have planted new trees.