Carbon does lose its usefullness as binding sites on the surface of the AC are "filled" by organics.
but a rinse in water while rubbing the carbon together exposes new binding sites for the dissolved organics to bind.
so rinsing weekly extends the usefullness significantly.
once the binding sites are all full (via adsorption) you can only 'reactivate' activated carbon by heating to 550 degrees for 20 mins. (this is a standard lab procedure for burning off Volatile Organic Compounds (VOCs) and this heating also 'reopens' the pores in the carbon matrix.
but its not really that practical.
I use mine and rinse weekly and only replace when I add meds.
it ends up being a great media for bacteria to colonize and it also acts as a good mechanical filter as the bag gets dirty.