The activated carbon is used to remove chemical such as medications from the water. It will lose its effectiveness after a few weeks (1 week is a bit short and probably the manufacturer trying to make more money out of you!!).
However you don't need to have activeted carbon in your filter all the time. I only ever use it when I actually need to remove meds from the water.
I would get some spare pads, then keep them handy for when you need them. until then use the space in the filter for extra bio or mechanical filtration.
