Carbon will become exhausted over time, it can only hold so much contamination per dose of media. If you have elevated levels of contaminants with something like yellow water or other foreign toxins, it will exhaust quicker compared to everyday use.
Carbon works fast and should be changed out every 7-14 days (maximum) in a saltwater aquarium if being used on a regular basis. If you're not using carbon regularly but instead are trying to target a particular toxin or contaminant, 3-7 days is usually enough and just replace it with a new batch of carbon to ensure effectiveness thereafter.
Leaving carbon in your tank longer than necessary could result in the leaching of removed contaminants and nitrification if left long enough for bacteria to grow on the media granules.
Carbon can be used inside a fluidized media reactor for maximum effectiveness. You can also just use a media bag for a more passive approach. While a media reactor will increase contact time and effectiveness, carbon is fairly effective in a media bag when placed into an area where water can flow through it. In either case, just be sure to change the media out frequently for the best results.