I clean my sponges about every 2-3 weeks, but really on an as-need basis. I can tell from the water level in the filter and if I wait too long and they get too clogged the sponges will lift out of the water.
I agree, don't run carbon unless you have to. However if you do run it I would replace it every month or so depending on your stocking density. Don't rinse carbon once you start using it. It can release stuff back into the water and chloride will reduce its lifespan.
The little balls sound like some sort of phosphate media, ion exchange resin, or purigen. Definitely not biofiltration unless they are the biopellets (almost pea-sized) that get eaten by bacteria.
