first, i sell softeners....
depends on lots of things...
1. grain capacity
2. hardness setting yours is set on
3. water usage
if the grain capacity is say... 30,000 grains
and your hardness setting is at 20...
for every gallon, subtract 20 grains from the total capacity.
now, the softener will regenerate when 2/3-75% of it's capacity are used up... so we'll say ~20,000 grains can be used.
20,000/20gpg(grainspergallon)=1000gallons can go through before it regenerates.
avg person uses 75 gallons per day, so family of 2... = 150 gallons a day.
1000gallons/150 a day=6.667 days
in that senario it will regenerate every 6.667 days
usually they are preset to regenerate at 2am or something, so if it gets close to 2/3 it will prob regenerate that night.