First let’s clarify the salt in the brine tank is used when a water softener goes through regeneration. Regeneration if the unit has been sized properly for the family living in the house it should go through regeneration about once every seven to ten days.
In San Antonio which has very hard water a system should use between 12 and 18 lbs. of salt per regeneration for average family. This would apply to city water not well water systems. Water softeners today work off gallons going through the system.
A house with one person with a 30,000-grain unit will go through a 50Lb bag of salt every five to six weeks. A house with four people if softener is sized correctly a 45,000 or 60,000 grain unit a little more than a 50Lb. bag a month.
If you are using multiple bags per month and you have city water probably your softener is not working properly, or it is sized incorrect for your household.