My bog is 12 inches of small river stone. Under that is my (2 inch) PVC manifold which are two legs about 10 feet long. Each one has a vertical clean-out stack at the end with a screw-in cap.
Maybe once every two months I'll notice the water flowing from the bog has reduced a bit. I'll shut off the pump (which is in the pond), open a clean-out stack then turn the pump back on. Black water will shoot out for a couple seconds, then it turns clear. I do the same for the second stack. Afterwards my water flow from the bog to the pond returns to normal.
So, I'll guess what I'm doing is flushing the manifold which is preventing any buildup.
It's really the only maintenance I perform.