Consider SeaChem's Purigen in place of running carbon. Do a search and you'll turn up additional information on Purigen. I speak of Purigen because it's a synthetic adsorbent (i.e. will not leach contents back into your water).
Your question though... replacement of carbon will get varied response here. Some will say don't run it at all, other will tell you after a water change for only a week, and yet some will say run it all the time, changing every 2 to 4 weeks.
I only ran carbon (GAC
to be exact) right as my cycle was finishing to "polish" the water (and I only ran it for 4 weeks). I've been using Purigen ever since with no issues. I replace/recharge it every 10 to 12 weeks.