If you're in a soft water area depending on your TDS you only need DI to bring your TDS down to 0.00,if your changing resin so often you need to look into better quality resin,I'm in a hard water area where my TDS from tap is 260 and through my RO brings it down to 0.10 and the resin brings it down to 0.00,change the resin every six weeks,it's only a 10 inch vessel and am getting through 500 litres a day of purified water,I think your resin quality is to blame.

,if you have a RO the filters might need changing after 6 months but not every month,like I say check your resin,I often had dodgy stuff!