How often or should you change out Tranny fluid

I tend to stick with every other oil change. Since I run synthetic with either 10k/1yr with the small vehicles and 10k/2yr with the RV somewhere in the 20-25k range for the trans. Make extend that to 3 or 4 if I put LubeGuard Red in. Typically just pull the plug and refill, can't remember how the 4L80E takes but our Escape takes 3.5-4.