As often as you think is needed. Some people will say that you have to clean it every week to keep the tank absolutely pristine and others will say over that and not to disturb it. With a ten gallon i would just rinse it out to get any crap out once a week, but i wouldnt change it until a month or more has passed. personally, I've learned that you don't even need carbon, and that just a sponge filter will suffice, since all the carbon does is remove chemicals from the water.