The answer is it depends. A tank should be pumped every three to five years. (More often if you have a garbage digester.) The frequency of cleaning depends on the size of the tank and the use it is given. The owner or a qualified technician should inspect the tank every year.
There are many different sizes and shaped tanks being used, however most tanks being installed today are 1000 Imp. gallons or larger. Sludge accumulation is approximately 80 gallons per capita per year or, 320 gallons per family of four.