Carbon is really only needed for a few things. It's good at removing medications, smells, some forms of cloudiness, and is also good for helping with chemical warfare with corals. It will usually last from about 1 week to probably a month depending on how much you use and how good of a quality it is. That being said, 95% of the time you don't really need to use it.
Mebbid's Fishtopia: Various builds
"The simplest explanation for some phenomenon is more likely to be accurate than more complicated explanations." -Occam's razor