I like Al's advice: "Wait until they are completely dry, then water them the day before."
As John mentioned, many factors determine how frequently the trees should be watered. When the containers start to feel (you have to learn how to gauge) light, then water. You do not want the growing mix to be too dry as it becomes hydrophobic. If this happens, give it some water, wait five minutes, then water as usual.
Also, the leaves can be used as a gauge. When the leaves wilt during the heat of the day but don't spring back when the sun starts to set... water the plant.
~james