How often or should you change out Tranny fluid

I changed out my trans fluid in our 2012 E-350 Class C switching to pure synthetic at 16,000 miles (Ford says to change it at 30K), as we were about to depart on a 7K+ mile trip. Switching to the synthetic reduced my trans fluid temps by 10 degrees across the board. I expect to go 25,000 miles before changing it again.

:C