Depends on the size of the filter, and what you want to accomplish with it. If you want to keep your water polished, then run the sponge and carbon. Clean the sponge weekly and replace the sponge and carbon monthly.
If you aim is to increase the denitrifying bacteria in the tank by means of additional LR rubble, then I'd suggest using a larger AC filter and modding it into a fuge. (very easy to do) You will need to light it though in order to keep macro growing in there.
I did that with the AC 300 and sitll have an area where I can run a poly/carbon pad, so I have the added biofiltration and the poly/carbon pad to clean out the gunk. This pad is replaced weekly btw.