cabon doesn't kill anything, it adsorbs varous odor and color causing chemical compounds,
do a quick google search and you will find 100's of pages explaining how it works
so I don't see how it would help on hitch hikers...
but how long and how often and how much carbon to use is an opion that is varied as there are stars in the galaxy
best advice is, if you think you need to run it - then do
FWIK running carbon will not do anything but good to your tank system, so when it doubt, run it!