I only use it if my water is yellowed and a change doesn't fix it or if I'm removing meds, detritus etc. I also use it for a week or so after I add a new coral , softy, anem etc. to remove toxins.
I run it continuously. After doing tons of research on it I have found that it's more a matter of personal opinion than anything. One tip though, if you're mixing softies/lps/sps in one tank or have alot of sps in a tank carbon is a good idea to run all the time because the corals sometimes have "chemical warfare" between them and release toxins into the water that the carbon will react with and make it safe again.