Thanks to all those who offered their thoughts on how frequently to
clean a septic tank.

In addition to reading replies I have also done some research. It
appears that, yes, tanks can be of different sizes but essentially they
all operate on the same principle. This principle is as follows:

1. All your waste flows into the tank via an opening fairly high up the
side of the tank.
2. The "heavy stuff" falls to the bottom.
3. The outlet pipe is roughly the same height as the inlet pipe and
usually on the other side of the tank. Only the "light stuff", i.e.
"clear" liquid should ever flow out of the tank through this pipe and
into the soakaway / gravel conduits / "herringbone".
4. A good level of bacteria in the tank works on the "heavy stuff" at
the bottom of the tank and assists in reducing it.
5. However, sooner or later the level of the "heavy stuff", i.e. the
"sludge" at the bottom of all tanks, WILL build up sufficiently to reach
the outlet pipe UNLESS you intervene and reduce the sludge level, i.e.
have it all sucked out.
6. If the sludge DOES get as high as the outlet pipe and starts to exit
the tank via this pipe it will move off into the soakaway land. No
problem will manifest itself for some time. Eventually however the land
will become clogged with the sludge and your only option then will be to
build a new septic tank on a separate and clean piece of land.

So, from all of this, it seems pretty clear to me that one needs to
devise a way of checking on one's sludge level!

Does anyone do that? I'm thinking of making a "sludge-detector" by
taking a long pole and fixing a fairly large flat plate to the end of
it. If I were to then slowly lower this down through the top level of
"clear" liquid I ought to be able to feel when I hit the sludge level.
If it is way below the outlet pipe, then all is well.

What do you think?

Yes, I know I should just make the "detector" and go test it . . . but
who wants to go outside in this awful weather and start poking around in
"heavy stuff" if it might be a complete waste of time?

Eddy.