You don't need to change the biowheel at all. That is a place for the NB bacteria from your cycle to live.
You may find in a year or two that your biowheel stops working however. Mine eventually stopped turning and the bacteria died. So I just took the biowheel and threw it out and have now put some sponge and ceramic rings in there instead.
Just make sure every few months you take the filter apart and clean the impeller and the little grooves that the biowheel sits on and it should still keep working.
The only thing you need to change would be the carbon inserts. You should change those every 1-2 months I believe. I don't use carbon in my filters unless I need to remove medication from the tank so I'm not sure how long it lasts in the filter.