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Ditto, or clean it whenever it's noticeably dirty.
My recommendation is almost universally to remove and not use sponges....they're a dirt magnet which can be good, but unless you can stay on top of cleaning them that also makes them a nutrient reservoir and algae generator.
For most people that means ditching the sponges to cut down on excess nutrients, but YMMV. Just don't try to fool yourself or your tank....clean em or don't use em.
If the sponge is really easy to get to (nothing to move or remove to get to it) and you have a spare, pre-cleaned, aired-out sponge to go in its place, then I'll give you a chance at keeping it clean. Make is easy on yourself in the long run!
FWIW, your dirty sponge can be cleaned overnight (or less, maybe a few hours) in a 10% bleach solution - one part bleach (plain-jane laundry bleach) to nine (or ten) parts tap water. After cleaning, rinse super well in new water and don't put back in the tank until the sponge doesn't smell of chlorine. This may imply that you need to have more than one spare on hand, so one can be in operation, one can be getting cleaned and airing out and the third can be sitting around ready for service. BTW, airing out is ok...should be no requirement for dechlorinator or to do it all with rinsing...chlorine doesn't hang around when presented with open air. Give it a day or few, as needed.
Good luck!
-Matt