How often should a single purpose tank be cleaned?

From field experience, based on a family of four with average use, a single purpose septic tank (systems which accept waste from the toilet only), will probably function well for five to seven years before requiring a pump out.

How often should a multi purpose tank be cleaned?

Multi purpose septic tanks (which accept toilet as well as sink, bath, shower, trough and basin waste), should be pumped out sooner, between three and five years, to keep the system functioning smoothly. Ideally, pump out your tank before it blocks up as prevention is definitely better than cure.