I don't seem to develop very much cake. But I scrape with the spoon-end of a pipe nail after every smoke, and ream maybe twice a year.
From what I read, there are two kinds of cake. One is soft, almost spongy, and can almost be peeled out of the bowl. The other is rock hard and can chip or crumble if scraped without due care. The theories range from the type of tobacco used, and improper packing of tobacco, to smoking cadence and tamping (or lack thereof.)
I've seen pipes with that outrageously thick cake that Phil describes. I think that's due to negligence or sheer laziness. Why would anyone who is even semi-conscious allow cake build-up to reduce the capacity of their pipes so drastically? Could it be that they know something that we don't -- like, say, it tastes much better? I dunno.
I find myself sitting idly on the line dividing past and future,
as if I could kill time without injuring eternity. -- Thoreau