Here is a septic tank that has not been pumped out for many years. There is about 2' of scum that has floated to the top. And about 2-3' of sludge at the bottom.
The Ministry of Enviroment recommends every 3 to 5 years to have your tank pumped.
Examples of a Dual compartment
LOTW Septic Barge will pumpout septic tanks, holding tanks or outhouses on Lake of the Woods