My TDS is 60 and I pump straight through DI vessel up the pole.
An 11ltr vessel will last me about 2 months, using roughly 250 ltrs a day.
I let the pure water creep up to about 8 - 10, then change resin.
You can always use a second DI vessel to save new resin life a little longer, something that I will be doing in the not to distant future.
Hope this helps