How long do you let your carbon filter go before replacing? Clean or replace?

What is the normal life span of a carbon filter? I took it apart and cleaned it as best I could but there is a paper filter that keeps the carbon particles from being sucked up its the most inner filter or the last stage. Seems to be awfully black. Can you buy a replacement one of those anywhere? I know I can get new carbon and a new pre-filter. Just wondering about that thin junkie looking paper filter. Or do you just replace your whole unit?
 
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