HOW OFTEN SHOULD I HAVE MY SYSTEM PUMPED

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 to much sludge accumulates, solids could possibly clog your pump and spray heads.  

If your Provider does not perform this test during each inspection, you may want to consider finding a different Provider.