According to Professor Mary-Louise McLaws, professor of epidemiology in healthcare and infection at UNSW, you should:
REPLACE YOUR SPONGES EVERY WEEK (say what??)
Apparently there are more germs in your kitchen, than your toilet.
It is recommended to use disposable cloths such as paper towels (I use pinoclean wipes for anything on the floor).. or different coloured sponges for different areas of your kitchen. (In my experience that never works, because hubby and kids just don’t care what sponge they use).