If you shake the bottle and move the resin around, then you will get more litres of water per litre of resin.
Also if you run 2 di tanks in line order then the resin lasts much much longer.
My tds of tap water are 72, with just 1 di tank, I was getting about 2 months of work from a 25 kilo sack of resin, with 2 di tanks this has now gone to 5 months and still going strong