Cleaning Fluval 305 type filters?

Hi, how often do people clean out fluval 305 type filters? The reason I ask
is that mine doesn't seem to need doing for months at a time, but when I do
it, I find that there is a lot of dirt in the ceramic biological part of the
filter which must have been pulled through the filter sponges and its just
sitting on the ceramic nodules.

So, could it start being blown back out of the filter as it becomes clogged?