If the salt in the brine tank gets too low, your softener won't have access to salt when the system regenerates, and you'll no longer have soft water. You should try to add salt at least every 4-6 weeks, depending on the size of the brine tank and your family's water usage. Make sure the brine tank is at least 1/3 full at all times.
Equally, you shouldn't add salt higher than the maximum salt level. Too much salt in the brine tank can cause salt bridges, which is equally damaging to a water softener's performance. If your brine tank has a big layer of encrusted salt across the surface, you might not even know when you run out of salt below. The resin bed won't have access to brine, and the system will stop producing softened water.