Martin was trying to simplify the topic. You can't really give a number, like 15 ppm, and say anything under that and the RO is working ok, and anything over that it is not (this would not be news to Martin).
RO membranes remove a PERCENTAGE of the TDS in the feedwater. This is called the REJECTION RATE. The higher the rejection rate, the more pure the RO water will be. The best residential membranes on the market - Filmtec - are spec'ed at 98% or 99% rejection, depending upon which membrane we're talking about. So if your feedwater is 250 ppm, only about 2% of that, or about 5 ppm should make it through to the permeate. If your feedwater is 2800 ppm (and yes - we have a customer with water like this), your perfectly functioning RO membrane would produce permeate at 55 ppm.
The lifespan of sediment filters depends upon how fast they clog or start to grow algae.
The lifespan of carbon blocks is determined by:
*how fast they physically clog, and
*how soon they start growing algae, and
*what's known as their chlorine capacity (stated in gallons). If done according to industry standards, the chlorine capacity is the number of gallons run through them at which they remove 50% or more of the chlorine. So a reasonable rule of thumb is change them out after 50% or less of the chlorine capacity. The devil is in the details re the chlorine capacity of carbon blocks. Most people aren't interested in the details here, and most suppliers couldn't tell you the chlorine capacity of the blocks they sell. That's what results in recommendations to change carbon blocks @ 6 months.
Russ