The owner's manual should answer the "how often" question. To get all the oil out of the rear end, I have always removed the rear cover, drained the oil out, clean up the lid and mating surface, new gasket and then filled it back up until the oil reaches the filler hole. I have only found the need to replace the differential gear lube after I had differential apart to replace a bearing or something. Of course if the vehicle was flooded or something then all the boxes will need an oil change.
hope this answers your question.
