Unfortunately there is no easy formula for determining how often your trees will need to be watered. Just as our personal water requirements vary dramatically depending on our physical condition, size, environment, and the weather, so too will your trees. Fortunately, we can offer advice about how to interpret your tree's watering needs. Signs that a tree needs water are: wilting, followed by browning off or 'crispy' leaves and leaf drop in severe cases. Signs that a tree has been watered too much include yellowing leaves, rapid leaf drop, poor growth, and spotting of the leaves. However, this can vary depending on the type of tree, so it is best to dig a small test hole 18" down to see how moist or dry the soil is.
Underwatering is easier to correct than overwatering if caught quickly, so frequent monitoring is important. Newly planted trees may need to be checked every couple of days in hot weather, and must be watered immediately at the first sign of stress. With good care and excellent growth in the first growing season, you may not need to water younger trees much if at all in the second, unless we have a prolonged period of drought. Trees with girth sizes in excess of 20cm will probably still need supplemental watering in the second season, but less frequently than the first. In both cases, do remember to keep an eye on your trees for any signs of dryness or stress for the first two growing seasons after planting and if a severe drought in the third season.
If you do still feel anxious about watering correctly, 'Tree gators' are an inexpensive solution. These swim-ring type bags are designed to hold water around the base of the tree, slowly allowing it to seep out through tiny perforations over the course of several days. You would probably need to fill your 'Tree gator' once to three times per week in the summer months depending on the temperature and type of tree. They are available through Majestic Trees.