our water starts at 450ppm, then after RO comes out at 012-014ppm,
we have a 1lt di on the end that lasts us upto 2 weeks. would last longer but we only work with the water on 000ppm, unlike some people that work upto 010ppm.
had one bag of resin last us over a year now,
our filters are changed every 6 months and membrains have been in fro 2 years now,
maybe look at changing filters/membrains too?? or flush longer as you water is lower than ours but coming out higher than ours!