I don't agree on routinely changing every 6 months.
Number one, it's costly if your on a moderate income level.
Second, if it's working what is there to fix?
A good TDS meter is the only real way to tell if you need to change anything. Readings of 0-5 are good. Above that, you may want to look at what the actual number is to tell what might need to be replaced. To me this makes more sense.