When setting up your water softener you usually have a choice of salt usage settings. The higher the salt setting the more "powerful" the force is on the water softener resin and therefore it will more completely remove the hard minerals from the water. As you lower the salt setting the force decreases and starts to let more hard minerals pass though the softener.
Softeners are usually assumed to be set at 15 lbs per cubic foot (ft3). At that setting, your softener will deliver the softest water (<1 p.p.m.). As you lower the salt setting the amount of hardness that can pass though the softener will be higher (2-3 grains) but the amount of gallons water you can run through per pound of salt used to regenerate the softener will increase.
For instance if we take water with 10 grains of hardness and run it thought a 15 lb regenerated 30K softener, we could expect to get about 3000 gallon or about 200 gallons per pound of salt. If we take the same softener and regenerate it with 6 pounds of salt, then we would make 2000 gallons of slightly hard water or about 333 gallons per pound of salt. For residential application having up to 4 grains of hardness would still be considered soft water. The only time you would use a 15 pound salt setting is for applications requiring less then 1 p.p.m. (1 grain - 17.1 p.p.m.)