I run carbon ALL the time (24/7/365) just like Cartman. I don't know why anyone wouldn't. IF any of you "true" reefers out there change at least 10% of your water once a week; there is NO reason you shouldn't run it all the time.
For those of you who only run it periodically or when your water starts to turn yellow/brown in colour, you should consider the pros/cons of using carbon more often if not--nonstop. There are more pros than cons.
Yes, carbon does remove trace elements out of the water; BUT there IS no proof that alot of those trace elements are truly necessary for coral skeletal growth and as a matter of fact IF you change your water once a week like a good little "reefer" those elements are automatically replenished.
Yes, the carbon adsorbs which is what it's suppose to do will stop working after a given period of time and it will release those "metals or organic solids" back into the water BUT ONLY if the pH changes from one extreme to the other. In other words, the carbon will release the bad stuff back into your tank BUT only if your pH goes from 8.2/8.4 all the way to 6.5/6.8 which if that were the case you'd have bigger problems on your hands like DEAD FISH OR CORALS/INVERTS rather than activated carbon issues.
Carbon removes that bad smell of the tank water and solids; gives the water a nice clear pristine look (something we all appreciate in the end).
Truth...when should you run carbon? All the time.