Using greywater for irrigation so that regular tap water need not be used is an excellent way to save water. The amount of greywater captured and available for irrigation is a function of the greywater system itself and how much water is used indoors.
Simple greywater systems that utilize a bucket in the sink and shower or a hose hooked to the bathtub drain can save a maximum of the capacity of the bucket or bathtub each time it is filled. A carefully designed greywater system that is connected to sinks, tubs, showers, and the clothes washer can create approximately 35 gallons per capita per day or 12,775 gallons per capita per year. A family of four could potentially create an additional 50,000 gallons for use outdoors. This of course assumes that the system works perfectly, all possible fixtures are connected to the system, there is sufficient storage, and very little water is lost in the filtration process.
More realistically, a family of four with a well designed greywater system with nearly all potential fixtures connected could save 30,000 to 40,000 gallons of water per year and in many cases it may be less than this.