Martha says to do it every 2 weeks! I do it when it starts falling apart, and I use this awesome sponge which is the best sponge ever and it lasts forever:

http://www.amazon.com/gp/product/B000GLPPLW

I do run it through the dishwasher or microwave to disinfect it, but apparently not often enough.... martha says to do it every other day!

How often do you clean/replace your sponges?