My bog is only 4 feet by 6 feet (and four feet deep) - there's really no "further away". The centipede goes from one end of the bog to the other. The flow is constant and steady from one corner to the other. In the spring before the plants start to fill in, you can stand and watch as the water moves over the entire surface. Once the plants get going, it's more of a "let's make sure we don't block the water" kind of adventure all summer long.
We've never cleaned out our bog. I've done a few experiments - digging down into the gravel, dropping a pump into the bottom of the snorkel - looking for signs it needs cleaning. Nothing. Gravel is relatively clean, water is crystal clear from the bottom. If we get sediment at all, it's on the surface of the bog - fine particles that fall from the sky, like pollen, dust, etc. Causes no issues so I leave it be.