IntelliTec TDS Meter

[TTT]

I just picked up this TDS meter from Wal-Mart for $15. I read the discussion thread that was posted about a week ago and thought I would try it. My reading on my RO water is 60 ppm. This seems high and was wondering how often people are having to replace their filters on their RO units and at what reading triggers your replacement? Mine has been running for 5 1/2 months now. It was never tested from set-up because I looked around and couldn't find anyplace that sold TDS meters.

Thanks in advance.