I haven't used carbon for years. And I do (maybe) two water changes per week of about 5% or so...usually it's just once per week. I do NOT ever recommend doing such massive water changes unless you have a very large stock of fish and little plant mass. If you need to rely on massive weekly water changes, you are keeping your tank in a state of flux and will probably experience algae issues that will take a really long time to go away. At the very least, you are doing a ton more work than you need to do, and if you need such enormous water changes to keep the tank healthy, then there is a serious underlying issue.
I'm certainly not advocating that you go my route. But, I wouldn't necessarily be aiming to do huge water changes either, at least, down the road. Especially if you plan on having a heavily planted tank.