I recommend you nuke a WET sponge every 2 days – it’ll look better AND harbor far fewer bacteria – My sponges last 1-2 months. This recommendation came from a young chemistry student I met.
WSJ: Bevy of Bacteria
A new sponge fresh out of the package is likely to harbor very few bacteria, says Dr. Sukol. But after a couple of weeks of normal use, sponges can be among the dirtiest objects in the home. Studies published by the University of Arizona over the past decade showed that a sponge can harbor 10 million bacteria per square inch and can be as much as 200,000 times dirtier than a toilet seat.