My Spectapure has in line and out line at every stage. I do the Sediment and carbon block every 6 months, the maxcap when it shows 1ppm usually two years, and the silica buster when I shows 1ppm. That's usually at the 3 year mark. These are both the high capacity. I change my membrane when it goes below 95% rejection. In my system that shows roughly 5ppm on the meter when coming directly from the membrane. So far that has been at the five year mark. My unit has self flushing so I flush after and before making water. I have never had my final stage above 1 ppm in the whole time I've owned the unit.
Your mileage may vary as my TDS coming in is 90ish and my psi is 80 into the house. So I get probably much longer than most. We do have sandy sediment so I HAVE to change sediment filter at 6 months or it gets too sandy.
Edited to add this is why I bought the best RODI I could. The meters have saved me tons of money, no guess work on which filter needs replacing.
