How longs a piece of string?
Sorry, what I mean is it would really depend how much water you were using per week. My water in to my van mount comes from merlin ro at about 30 tds. I have an 11 ltr di and use approx 500 -700 lts in a week. I bought a 25ltr bag of resin at beginning of year and have one refill worths left that I shall probably use next week. Hope thats of some help to you.
Simon