my tap is 98-104ppm (it varies) i use RO and a 6.5ltr DI vessel to bring the 1ppm after RO down to 0ppm and de-ionise the water.
5ltrs of tulsion resin will last me 18mths ish. (25ltr bag is £74.50 delivered),
prevoiusly to having RO, i was DI only and one 5ltr fill lasted a week. it didn't take me long to see how fast an RO would pay for itself. i do have a small DI though and the recommended size for DI ONLY is minimum 11ltr vessel.