How much salt a softener uses in the brine tank depends on how often it is set to regenerate. Regeneration frequency should be set by the gallons-used-per-month to achieve the desired softness of the water that you're treating.
If it's doing its thing to suit you, just check the salt once in a while until it gets way low in the brine tank. Then, take a clean broom handle or something to stir the salt left in the bottom of the tank to break up any hidden salt "bridge" that may have formed in the bottom, and just re-fill it.
Mine holds a couple of 80-lb bags of rock salt. It comes in smaller bags, if you don't want to lift that much at once. It's a little cheaper by the larger bags, though.
Also, I tried those salt pellets, which are just salt grains compressed into pellets, and they tended to clog up my softener, so I stick with the rock salt. I have real hard water, so I have it set to regenerate frequently, and I usually have to add the 160 lbs of salt about every other month or so.