Grains are a small measure of weight. So suppose you have a 30,000 grain water softener. A 5 pound salt setting is satisfactory for hardness 5 grains to 39 grains. An 8 pound salt setting is satisfactory for hardness 40 grains to 100 grains. A 12 pound salt setting is satisfactory for hardness over 100 grains.
Timer controlled systems have the advantage of being simple.
They have the disadvantage of regenerating whether the unit needs regenerating or not. Demand controlled water softeners have a small computer in the head and monitor the amount of water you use.