i'd change your resin when your TDS reading goes above 5 although some lads on here prefer to do it earlier and some go up to a reading of 10,i change my filters every 6 months or so but to be honest if your not needing to change resin after a year your filters should be fine.i'm guessing you don't use much water.

also i have changed the upright 10" filters already thought you had to change them every 6 months

ok thanks for that kirky my reading has'nt gone over 000 would be concerned using water with higher tds reading