We do 30-40k with our Duramax, synthetic fluid, and it deals with the abuse of towing, plus my wife's lead foot. Tranny still shifts great with around 170k on it now.
On my gas engine stuff, I have ran longer intervals, like 50k no problem. Only one had trans issues, but that was at 150k miles, plus the car ran 12.80's on drag radials. Basically I beat on that tranny good.
Both are just draining pan. But if you want to get it all out, pull the line off after refilling, have someone start and put it in gear, watch the color of the fluid pumping out of the line into your catch container. When the color goes from dark, to the new light color, you're good. Then just top it back off.