Replacing Carbon in Rainsoft EC4

The company who installed my rainsoft has been calling all summer about the under the sink filters needing to be changed and the carbon needing testing/possibly replacing.

I am changing the under the sink filters myself for much cheaper than they want to charge.
My question is: How often does the carbon needs to be relaxed or how do I know if the carbon needs replacing? If it does, how do I go about doing that?