Fish: how much of the Beneficial bacteria lives on the Filer Media?

The bacteria will colonize nearly every surface in the aquarium, on the filter, the substrate, even the gl. As long as you don't over clean the tank, the bio filter should recolonize in a week or two. If possible, if you have a HOB filter that you can replace the carbon and the pad separately, you can rotate changing the two, (i.e. change the carbon now, and in two weeks change the filter pad), and just keep rotating them, you will have minimal effect on your bio filter as the bacteria will colonize the microscopic holes in the carbon too.