How often does the unit need to be replaced? Or, what is the life of this unit in areas with hard water?

Hi, typical life span is roughly 15 years.

Hard water does not affect the life of a w/s. Normal regenerations will keep the resin bed flushed. I've serviced units that were 20-25 years old and were still going strong. Usually the valves will break down well before the bed goes bad. The thing that ruins resin beds is iron. That will require extra cleaning with resin bed cleaner and if that is done on a regular basis the bed will still last many years...