You should have done it at least twice by now. I changed my filter and pan fluid a few months back, at 30K. The following week I took it in for to have all the fluid fully flushed.
To my surprise magnet in the pan looked 100x'* better then the pan magnet in my truck with the same mileage. When I did this service to my truck I did it the other way, having it flushed first then I changed the filter 1K miles later myself. The magnet was pretty ugly compared to the Bonnie'*
Here'* what the magnet in the truck looked like at 31K and after having the fluid flushed at 30K. The magnet in the Bonneville was really clean (sorry no picture)