You do not need an exact amount of salt in your brine tank, however, you need to replace salt when you can see water laying on top of the salt. How often is that? It depends on how often your softener regenerates (runs) and how much water you use. A general time frame is every eight weeks.

On the Other: Other thoughts

Make sure that there is at least enough salt for the regeneration process to take place. Keeping the level at least half-full will give you a consistent performance. Checking the salt keeper every 30 days never hurts. The amount of salt in the tank has nothing to do with the amount of salt used. Generally, the same amount of salt is used each time the softener runs through the regeneration process. Processing the water through the salt removes the hard parts of the lime and calcium in the water. On average, this process will use approximately 60 pounds of salt each month.

Bottom Line

Keep the salt level of your water softener at half-full in order to have good operation. The more water you use, the more often you need to recycle the softener and the more salt you need to add. The safest thing to do is to check the salt keeper every 30 days.

Source:

Watts.com: Water Softeners