How long do carbon filters last?

My friend has a 4' carbon filter attached to a 12" vortex. The carbon filter is on 24 hours a day along with the fan taking out the air. About how long is the carbon filter supposed to be good for if it is used continually without ever being turned off.

with the pre-made units, they last around 12 - 18 months. a home made unit i would change every 6 months or so.

A 12" Vortex?? Holy shit, how big is his grow room?