I installed a drain on my pan years ago. I do a drain and fill (only about 3 quarts get changed this way out of the total 10 quart capacity) every 10-15k and change the filter every 30k (which changes about 4 quarts out of 10)
This method obviously doesn't change all the fluid but it keeps enough fresh fluid going in to maintain it. And its inexpensive.
Some will argue thats overkill but 3 quarts of fluid is about 15 bucks and a transmission is a couple grand.
Also depends how your truck is used. Lots of highway miles are about one tenth the strain as lots of city miles. Offroad and towing is obviously even worse. 2 people with the same truck and person A. might need fluid changed every 6 months and person B. could go a few years. Check your fluid often for any signs of discoloring or burnt smells.
Now I should note if Im working with a vehicle with unknown service record or im servicing it for the first time and there is over 60k on the fluid I change all the fluid ( on the ranger that means 10 quarts ). And then begin the partial drain and fill routine.
To me, letting trans fluid go 100k plus without any service is just insane if you plan to keep your vehicle.