Seachem claims it does not need to be replaced because the pores are naturally too small for organics to clog, but large enough to be washed to have the old bacteria remains removed. I think it can be rinsed and boiled (several times) to be rendered reusable. I would just avoid the use of any products because of the highly porous nature of pumice stone.
I wonder on its cost effectiveness, I'm guessing it won't be better than anything else in a few months. If pumice is cheap enough though, you could simply toss and replace like filter floss.