Activated carbon or AC is honeycombed with a vast network of minuscule channels that account for the material's filtering power.  As water passes through the labyrinth, organic contaminants condense on the walls of the channels.  It stands to reason, then, that the more “charcoal” in the filter, the longer it will last.  Activated carbon, usually in the form of granular activated charcoal, work best against organic compounds.

Sellers of water-treatment devices tout AC's versatility, sometimes claiming that an AC filter can remove everything that mars the quality of drinking water.  Not true.  An AC filter can't remove everything - microbes, for example.  Indeed, under the right conditions, an AC filter can become a breeding ground for bacteria. 

AC filters work best when they're allowed to work slowly.  The longer water stays in contact with the carbon, the more contaminants will be trapped.  For this reason, most large capacity AC filters come with narrow tubing which retards water flow to 1 to 2 quarts per minute, a stream roughly the diameter of a pencil.

The typical high-volume carbon filter cartridge is about 10 inches high and 3 inches in diameter and contains enough charcoal to treat about 1000 gallons of water.  Some high-volume filters mount under a sink and others can be set on the countertop.  Most dispense filtered water from their own faucet mounted on the sink or countertop.

Faucet-mount filters remove off-tastes and odors, but they are virtually useless at filtering out dangerous contaminants.  The problem is they are small and water flows through them quickly relative to high-volume cartridge filters with flow restrictors.  Activated carbon in such designs has little opportunity to do its job.

Pour-through carbon filters function much like a drip coffee maker.  Water is poured into the top of the container and it drips through a carbon filter to yield a few quarts of drinking water.  Pour-through filters work slowly.  Typically, instructions tell you to keep them in the refrigerator, where bacteria are less apt to multiply.   Water-treating capacity ranges from 20 to 100 gallons between filter changes.   Pitchers are able to remove off-tastes from that much water, but don't depend on them to remove health-threatening substances.

Save money; regenerate with hot water when you notice the reappearance of the off-tastes and odors that led you to buy the filter in the first place. Running hot water through the filter releases the adsorbed molecules (something filter manufacturers don't tell you). Flush with hot tap water until the unit is hot throughout, cool with cold water and return to service. This may be done as many times as you wish.

Recommendations
If an analysis shows that your water is contaminated with organic chemicals, you should install a high volume carbon filter.  Don't rely on “taste and odor” faucet-mount or pour-through pitcher filters to solve the problem.