I upgraded the trans cooler on my 97 F150 with an SD cooler.
I replicated the install in this link...
https://www.f150online.com/forums/ar...how-loads.html

I just had my trans fluid changed while the truck was in for inspection.
The shop drained the converter and refilled with Mercon V.

I had only done the cooler install about a year ago, but I changed the filter with the trans service.

Any thoughts on how often to change the external filter going forward from here?

I've been changing the trans fluid at 30,000 mile intervals.
I am thinking that the external filter change could be done at 15,000 mile intervals.
I tow heavy during the summer, my boat is about 6100 lbs. Usually short distances every weekend. During the fall, I tow the band trailer, about 4000 almost every weekend between Labor day and Thanksgivng.

The cooler works great, temps are about 30 lower than before I upgraded.

 

I don't see a need to change the external filter in between fluid replacements.