How Often Should My Septic Tank Be Pumped Out?
Technically speaking, when the solids occupy 35-50% of the septic tank volume, then the tank needs to be pumped. The only way to know this is to have the tank inspected, which a local septic contractor or pumper can do.
A general rule of thumb is every three to five years. The table below estimates a suggested pumping frequency based on the size of the tank and the number of people using the system. And once your tank has been pumped, it is not necessary to "reseed" it with anything. It will work just fine with the normal wastewater that enters the tank through everyday use.
Table from Pennsylvania State University Cooperative Extension Service (Robillard, 1990).