I suggest running it for 48 hours once a month. Make sure lots of water is forced through the carbon, such as a canister filter, and use a high grade GAC.
Another thing to keep in mind if you use carbon, use the better stuff like Black Diamond, as the el cheapo carbon can leach phospate back into your system!
I change it out weekly, and use small amounts. Wonderful stuff, but it does remove additives.
Carbon is more of a personal thing than a necessity. I recommend it, though, along with monthly or bi-monthly waterchanges. The fewer waterchanges you do, the more carbon can help you. It will help make your water look crystal clear when changed often. As though your fish are moving thru air
It also helps remove toxins produced by corals, and hormones from fish, amongst many other things.
I use Seachem's Purigen pads. They work great along w/carbon. It doesn't trap any additives, and is rechargeable using bleach, once the pad turns brown.
I use Carbon, phosban and Purigen 24/7, change the media every 10-days or so. I don't use a sump or fuge on a 29gal, so I don't have much choice to use all those DOC exporters to keep things stable.
Now if I could only eradicate my last patch of Hair Algea... it's only growing on the shell of one of my Astreas -- looks like it's got a 'fro. :wink:
I actually don't like Kent Reef Carbon as well as Black Diamond. I found that it doesn't keep the water as clear, unless you change it out every other day.
I run carbon continuously in my larger tank, but change it every five days. In my nanos, I drop a bag of carbon in whenever I feel like it.