How the math works
Softenry mathDays = usable capacity ÷ grains a day. Grains a day = people × gallons × (hardness + 4 × iron ppm). Usable capacity is the rating × 0.75 at an efficient salt setting.
SourcePenn State Extension's worked example: four people, 75 gallons each, 10 gpg is 3,000 grains a day, so a 20,000-grain unit regenerates about every six to seven days.
Questions
How often should a water softener regenerate?
About once a week is the usual target; every day or two means the unit is small for the load or set too hard.
Why does my softener regenerate every night?
A timer set to daily, a hardness setting far above your water, or a unit too small for the household.