Maybe things have changed with today's crushed coral but back in the day, there was no need to change CC gravel as the main ingredient in the coral was Calcium Carbonate. It was literally dissolving and releasing the Cal Carb when the water turned acidic (or tried to) which kept the hardness and ph high. The only reason I would change the coral that you have in the tank or in the filter would be based on it's effectiveness. It's not like a carbon that has a saturation point. In order for the CC to be used up, it pretty much has to melt away (which shouldn't happen for decades
) The best way to tell when to change your coral would be to first see if the ph rises when you first introduce it into your system. If it doesn't, you didn't add enough. Once you add enough to effect change in the ph, once the PH starts to fall again, time to add more or replace the c coral. The timing will be based on your tank and it's inhabitants.
In some of my saltwater tanks, you can see which natural coral decorations have been in tanks the longest. They don't look like what they did when they were first put into the tanks. Years of minute dissolving has smoothed over the coursness. All this time however, my water always tested great for the fish. It's natural
and how things work in nature.
Hope this better explains things.
BTW: I hope you don't have any decos like driftwood in with your africans. This is a known acidifyer and will definitely cause you to spend more time and money counteracting it's effects on your water