What's the tds after RO and before DI? If you're getting 95% rejection it should be about 22-23ppm
Mine is about the same and I have to change resin every 8-10 days (producing about 1600-1800lt/day) so if you produce (say) 400lt/day your resin should last about 32-40 days, but that's with a 6lt resin bottle. Adjust for smaller bottle.