My policy is to drop the fluid that is in the bottom of the trans every 30K. By doing this, I'm rolling over ~90% of the fluid every ~120,000 miles or ~6 years with my duty cycle.
The fluid does breakdown and lose it's shear strength over time. I made the mistake with my 2007 F150 of reading the owner's manual and believing it when it said to change the fluid at 150,000 for a non-towing vehicle. At 110,000 miles the fluid broke down and the clutches started to slip. I immediately researched it and changed out all of the fluid (a real pain of a job), I did it again proactively at 180,000 and the truck/transmission is still going strong at 205,000.
Since then, I've been dropping the fluid in the pan every 30K on all of my Automatics. Looking at the videos of what it takes to drop the fluid in the 10R's with the remote cooler, heat sheilds, exhaust crossover, etc....I'm springing for the pan with the plug. After the initial change, I only plan on changing the filter every 120,000 since they are basically just screens to keep the cats and dogs out. So, the pan with the plug will make the changes in between the filter changes a non event, not to mention avoiding all of the risk with getting the plastic pan to reseal after dropping it for the filter changes.