It is virtually impossible to over water a tree but as a guide 30 gallons a week is the minimum a 6’ tree needs. Watering regularly is better than just once a week and you may need to increase it when the weather is hot and dry. Water shortage can also be caused when the conditions are windy and dry. Watering is best done early morning or late afternoon to avoid evaporation.
Something that is almost as important as watering is mulching which helps to retain moisture and supress weeds. Just spread a layer of mulch,such as woodchips or leaf mould, around the base of the tree and this will help the tree to establish as it won’t be competing with weeds or grass for water and nutrients. Ideally it is best to mulch mid to late spring orautumn when the soil is warm not winter and early spring when the soil is cold and summer when it’s dry. Trees are best mulched to the radius of the canopy and keep away from stems to avoid them softening and getting diseased.
Top-dress in late winter with a general fertiliser.
Once a tree is established there is no need to water unless you experience severe drought conditions as it just encourages roots to come to the surface. However, if you do have to make sure you really soak the tree rather than just giving it a light sprinkling.