formula is
20,000tds per litre of resin - its a rough guide.
for instance, tap tds 100ppm you would get 20,000/100 = 2000ltrs of water per litre of resin.
using 2 di vessels in line means you can use resin that is part depleted to take the tds down from say 100 to 50 before going into the second vessel - resin normally lasts twice to 3 times as long this way.