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I change the sediment and carbon about every 6 months. I'm on well water here so the worst thing I'm getting is some mud in the sediment filter after a heavy rain. I change the RO once per year. I monitor my TDS and change the DI when I notice a rise in TDS readings. YMMV.
It really depends on your source water and how often you use it. Once your TDS starts to climb in your filtered water it may be time for a change. You can get a TDS meter pretty cheap. Once your level gets above 25 or so you may want to consider changing.