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 should be performed during each inspection.  Your Provider should keep a copy of the sludge levels so that he/she can better determine when to recommend pumping.  In turn, you should keep a record with the date the pumping was performed.  This way you and your Provider will have a better understanding of a general time line between pumpings.
Your Provider should 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.