Basic calculation is 20,000 parts (TDS) per litre of resin.
eg a bag of 25 litres will remove 20,000 x 25 = 500,000 parts (TDS). Therefore water at 100ppm you will get 500,000/100 = 5000 litres OR water at 200ppm you will get 2500 litres etc.
However on any given water this calculation may be way out as mixed bed resin is made up of 40% Cation resin (-) and 60% Anion resin (+) Diffirent elements of contamination in water are - or + charged, so depending on what - or + ions make up your water you will get up to 30% more or less through.