Not too much, not too little. How is a gardener to judge? Timing is everything. Knowing when to stint and when to flood makes the most of every drop.
Keep the goal in mind
Water with the goal of encouraging deep roots, fewer near the surface
Be generous the first year.
Deep, infrequent watering to encourage roots to grow downward
Reduce or eliminate watering in subsequent years
Water deeply or not at all
Whatever the watering method, always check to see how deep the water has actually penetrated. A moisture meter will give an accurate estimate of conditions ten to twelve inches below the surface.
Make sure the ground is thoroughly soaked by the end of spring rains, May to early June. Water less or not at all in late summer when perennials are hardening off for winter.
Choose the watering method that best suits your situation. Drip irrigation is great but expensive. Soaker hoses can be useful if the coils are placed close together for complete coverage. If you choose a sprinkler, water in early morning or while it's raining.
Use a mulch or grow a solid cover of plants
Mulches keep water from evaporating from the surface., but need to be renewed occasionally. Consider a solid cover of plants at least a foot thick instead of a permanent mulch. Plants draw water from deep in the ground and the extra shade and insulation helps to minimize loss from the surface.
Use a gravel mulch if plants need extra heat.
Late summer challenges
Very dry soil can resist wetting, even under a mulch. Water slowly, perhaps over several days, imitating a gentle rain.
In late summer, it may take too much water to soak completely but shallow watering means wasting water. During hot spells you may want to cover plants with straw or shade cloth or cut back leaves instead of adding water.