Probably 99.9% of your beneficial bacteria is found in the filter simply because it's the environment best suited for it. The rest of the bacteria would be found in your gravel and water column, but in amounts so negligible that they have little impact on maintaining the cycle in your tank.
That's why large water changes will not usually disturb your cycle, but changing out the filter pad will cause it to crash.
The only reason companies tell you to change out your filter media every six months is because a) some of the media in your filter is carbon, and it becomes useless after a certain amount of time (but does provide another surface area for your beneficial bacteria to colonise) and b) they want to make more money off you.
I have had a goldfish tank running with two filters for about two years now, and aside from a gentle swish every now and then in treated tank water, my sponges have never needed replacing