title basically says it all... I know they are good for hosting BB but they do get caked up with gunk and stuff like that... so how often should I change it?
I have never changed the sponge in my Eheim - I've been using the same one since Oct 2011 (when I bought the Eheim), I just rinse the gunk from it in a bucket of tank water every so often; it's still in perfect condition IMO
The filter floss is another story though; that I will change out every 6 months-ish.