Transmission flushes are not really worth it or necessary and sometimes a transmission flush may cause more harm than good.
Most transmissions can go 30,000 miles to 45,000 miles and sometimes even more without needing a transmission flush or transmission fluid change.
Although I change my transmission fluid and transmission filter after 30,000 miles just as preventative maintenance to help extend the life of my transmission.
If you're gonna flush the transmission fluid out you might as well just have the pan dropped and replace the gasket and transmission filter as well because it's not that much more work to do so.