I think it depends on how you change it and how much metal is in the oil. I change mine when I change the fluid in the transmission every 30k miles. My drain and fill port is on the top. I think the best way is to split the case drain and wipe down the inside then reseal and fill the housing. Mine did not leak nor did I want to cause a leak so I just pumped it out and refilled. No noticeable metal in the fluid so I probably wont change it again but my transmission still shows metal on the magnet so I will continue to change every 30k miles. To me it is more about inspecting than the fluids breaking down.
