How much carbon and how long?

Just curious how much carbon people use and how long they leave it in for? I have never used it and my tank is thriving but the water has a tinge to it so I figured id give it a try. I filled up a phosban 150 all the way up with cheaper Petco brand activated carbon. My tank is a 120g sps tank with aa 30g sump, with rock and all id say roughly 120g total water volume. I have a fairly large bio load and my nitrates have gotten a little high as of late (40ish). Corals are growing like crazy though so no impact there.
Is a whole phosban reactor too much for my tank? Also how often should I change it out? Obviously I don't want it to add to the nitrates.