How often do I need to add salt to the Brine Tank?
It depends on how often your system needs to regenerate.
The more your softener regenerates the more salt you will consume. As for the salt level in the brine tank, you can let the salt get down to the point inside the tank where you can see the water just above the salt; your will need to keep a 100% saturation of salt to water to have a good brine mixture. 100% saturation is where water cannot hold any more dissolved salt. When you see 2 inches of water above the salt, it is time to add more!
Generally, you will add salt to your brine tank about every 8 weeks if the system was sized properly before purchase and it is operating properly. Do not over-fill your brine tank though especially, if you have a float valve. The float valve is usually located inside a tube in the brine tank. Too much salt in the brine tank can have a crushing effect on the valve tube and can cause your float to raise and lower properly.