When to pump the septic tank
Septic tanks usually need to be pumped every 3 to 5 years, depending on the amount of water use. Generally, septic tanks fill faster as more people use the septic system. This means more frequent pumping will be needed with larger households.
During your maintenance inspection, the maintainer can measure the scum and sludge layer in your tank to determine the frequency with which you need to pump your tank. The septic tank should be pumped when the bottom of the scum layer is within 3 inches of the bottom of the outlet baffle or the top of the sludge layer is within 12 inches of the bottom of the outlet fitting. (See below.)
Single compartment septic tank anatomy