For carbon, I would think on average, it will be active for at least a few months (or you're not doing enough water changes
), so changing it every 30 days errs on the side of caution.
But as with my tanks, everything is equal, same corals, fish, salt, schedules, etc, just the actual two tanks (disp and sump) are different. But very different results. Not to say it's the glass, but I've pretty much thought of everything else.
I actually have a thread about this problem posted a couple of months back.
http://www.canreef.com/vbulletin/showthread.php?t=65821