Good answer for most cases but there's also the element of timing, that is, when to water. There's also the question of your reason for watering. For most plants, you can wait to water until the top inch of soil is dry. True for most soils. The roots are almost all in the top foot of soil, even top six inches, but not right below the surface. Use a digital tester--your index finger:)
If your goal is a prizewinning rose or dahlia you'll water often and a bit lightly with the purpose of making sure the plant never lacks water even for brief periods.
If your goal is a sturdy, healthy plant you'll water less often and more heavily.
And so on. A lot of this, alas, comes down to experience, which comes with time in the garden (years, really) but also with errors (dismaying but instructive).
I didn't water at all this year in spite of heat and drought, an experiment with my perennials. Phlox didn't like it, sweet autumn clematis rambled far and wide as usual, Japanese anemone really didn't like it, all the onions were fine. European gingers died to the ground. Likewise pachysandra. Meanwhile vinca major was fine.
The upshot will be shifting the garden to the toughest perennials over the next few years and trying a few new ones.
