Ya you pull out old solution which would be full of things from the film/paper processed.
How much effort are you willing to put into making sure your process is in control? If you're willing to jump all the hurdles then I bet you can keep replenishing forever. Go to the Kodak website and see if the info on process control is still there. I'm not sure it makes that much sense for the average person. Better to dump the stuff every so often but if you're doing processing every single day then maybe you want to stock control strips and get a densitomer.