Good question.  Typically, every 2 to 5 years, depending on daily useage.  Although this question does not have a definitive answer, there are tests that can be performed to determine when to pump.  A sludge test is performed during each inspection.  We keep a copy of the sludge levels so that we can better determine when to recommend pumping.  In turn, you should keep a record with the date the pumping was performed.  This way we will have a better understanding of a general time line between pumpings.
We also visually check the sludge.  If the sludge is a dark color opposed to a light color it is generally time to pump.  If too much sludge accumulates, solids could possibly clog your pump and spray heads.  To prevent unneccesary issues, it is important that you have your system pumped when recommended.  Be sure to have all three tanks pumped.