Carbon replacement - How often should you replace the carbon in a whole house unit?

QUESTION: How often should you replace the carbon in a whole house unit? My tank measures 10" x 54" and the water coming out is clear and odorless. I called one local company and they told me "never" unless there are problems or a noticeable change in the water quality and that back washing cleans the carbon. I've purchased parts from you folks before for my Autotrol systems. I am looking for an unbiased answer ? ANSWER:

*** They are on the right track, and much more honest then most companies ( most companies try to get you to do it every year whether it's necessary or not ).

The fact of the matter is, It Really Depends.

What is the carbon being used for?

What is it "removing" from the water?

And how much of "that" is in the water ( i.e. chlorine, iron, color, hydrogen sulfide odor, etc )?

And how much water is being used ( quantity or number of persons in household )?

** In best cases, the carbon can last 10 years ( in cases where you did not really need the carbon in the first place ).

** In worse cases ( lots of water used, and lots of iron, or chlorine, or hydrogen sulfide in the water ), every year.

Average is about 5 years.

see:

http://www.softnerparts.com/product/23A800912/

and

http://softenerparts.blogspot.com/2009/09/how-to-replace-media-carbon-birm.html