I've never replaced any aragonite/crushed coral/holey rock. As long as its still present, it will still buffer, but the capacity is limited by the amount.
I have crushed coral or limestone rock in all of my African tanks, never replaced any, and I'm sure the pH is stable in the 8+ range. Can't say for sure because I never measure pH, but the fish are happy, so no reason to change anything.
I would stay away from the buffers added with water changes and just add more aragonite until you get the buffering capacity you want. After that, all you need is dechlor for water changes.