Throughout this site, the following are used as guidelines for watering established plants:
These truly xeric plants can live with our 12 inches of natural annual precipitation and only need a winter watering during a multi-year drought, but they will thrive with a monthly watering. Overwatering will kill some of these.
These plants are adapted to intermittent deep watering with soil drying to a depth of a few inches between waterings. Watering frequency may be every couple of weeks during the active growing season and maybe only one winter watering for optimal care.
These plants need regular watering somewhat like a bluegrass lawn so that they never dry to depth in the root system during the active growing season, and need occasional winter watering to prevent root dessication and resultant plant death.