That's a difficult question to answer. We don't know your geographic area or climate zone. Are these trees newly planted or well established? What variety? Generally speaking, many trees benefit from deep infrequent watering. Arborvitae generally do best if kept moist, but not soggy. Some varieties require more water than others. If you begin to notice yellowing, it could be from too much water as well as too little.
When one of our cherry trees began to exhibit signs of iron chlorosis, the first thing we did was change that schedule to a fixed one that waters every seven days for more or less an hour. This year the trees greened up nicely and the growth was considerable. We just had our first substantial cherry harvest.
Your best bet would be to consult your neighborhood nursery, aborist or university extension office. They should be able to give you specific information for your climate zone and the variety you have.