When I bought the resin I changed over from a 10" cartridge to a 7 litre di vessel but the resin would only last the same amount of time as in the cartridge, I know I'm making more water now I have changed from backpack to van mount but not that much more though.
5 litres of tulsion lasted me a year when I first got the ro
I have just changed the pre filters and the membranes on the ro and it is coming out at 14ppm at the minute before it was around 35 ppm