I have a reverse osmosis and deionization unit that filters my tap water from 450PPM TDS to 0 PPM TDS. I have had it for about 3 months and my TDS reader is now creeping up to 3 PPM.
I was considering waiting until the TDS reader gave 10PPM to determine the amount of chlorine it in, and then decide on that result whether I should purchase new filters at 10 PPM every time in the future.
If I knew at what PPM of chloride causes adverse affects on the homebrew, I would be able to better estimate when I need to buy replacement filters.