The aquaone setup guides state that the ceramic noodles are between the course filrter medai (black foam) and the fine white filter wool.
This is how mine are set up, and while they get dirty, I don't worry too much about it, as it is not going to get through the fine filter wool below it.
As for how long they last, they will eventually erode, but this will take years - I actually use a mix of the cerramic noodles and plastic bio balls which have a larger surface are for bacteria to grow on.
(BTW, my tank is an aquaone AR980)