hi mate i use about 650ltrs a day i have a seperate merlin filter system in my garage with a 1000ltr holdong tank the merlin brings it down to 10 then my di vessels bring it to 00 change resin about 2 a year so i change my resin when it changes to 001 dont take any chances as the numbers can go up quickly, also the resin in the di cannisters can earn you thousands so dont scrimp to save yourself a few quid CHANGE THEM !!!!