If it starts to smell it is likely bad.
The common kitchen sponge is often more dirty than your toilet! That is how bad it is. Web MD and the Mayo Clinic classify it is one of the most germ filled item in your home.
One of the best ideas is to boil your sponge regularly.
I boil mine every couple days.
Also boil anything that contacts meat.
"Where will you drive your own picket stake? Where will you choose to make your stand? Give me a threshold, a specific point at which you will finally stop running, at which you will finally fight back." (Derrick Jensen)