How much water do water softeners waste?
You can easily calculate how often regeneration will be necessary if you know how hard your water is. If you are using city water, they should be able to tell you otherwise there are kits you can use to test your water. And any softener salesman should test your water as part of sizing your system.

In my area, the hardness is measured in grains per gallon and the value is 10.

My water softener is rated at 30,000 grains so I need to regenerate after using 3,000 gallons of water. I use about 100 gallons per day so I regenerate every month.

This link gives you an idea of how much water is used during regeneration for several different brands of water softener:
https://web.archive.org/web/20101007181800/http://www.watervalue.com/softener_regen_water_usage.html