It depends on the hardness of your water. The more hard water you have, the more salt you’ll need. Usually, the system will determine the amount of salt you’ll need. You just need to check the salt reservoir and make sure it’s regularly refilled.
If you must have an idea, here’s an estimate. If you have a family of four persons and the water hardness level is 7-10 grains, then you’ll need about 10 pounds of salt per week. You’ll need a bag of 40 pound salt per month.