Carbon works as a mechanichal and chemichal filter medium, but after a short time it becomes clogged and stops working. You might want to replace it every month. This will not necesaruilly mean an amonnia spike, since you also have other filtration material thet hold bacteria as well.
But, you might consider stop using carbon. It works for a very limited period of time, and unless you have something ugly in the water, you don't need it. Most people use it only to remove medicines left after a treatment, ot to remove tannins leached into the water by driftwood.