that is an extremely complex and open ended question but... For the most part, the industry has landed on 6# of salt per cubic foot, but most field guys know that 8# of salt will give much softer water. If you want I can post the bleed/efficiency curves, but in all honesty, 8# of salt per cubic foot of resin will give accaptable qual;ity and efficiency.
To properly answer your question, we need to know how big your system is, does the water have competing ions, like iron or manganese, how hard your water is, what kind of controller, and how many gallons you average. 60 gallons of water per person per day is common.
8 pounds of salt per cubic foot will yeild approximately 24,000 grains hardness removal.
Hope this helps.