OK confession time
I have a 200gpd Ro-Man unit.
Use around 3000 ltrs a week and I haven't changed the prefilters in the year that I've had it. 420ish TDS in and still gettiing 5 or 6 out as I always have.
If I get a year out of the membranes and prefilters at what they cost for the water I get out I am happy. Its probably down to what chlorine levels are in the water. As I said before the only purpose of the carbon filters is to get rid of chlorine.
In my defence I do flush VERY regularly.. shame on me.
Andrew