Put in a plug and change it when you change the oil, or every other time, and change the filter when required.
If you do that, you get 50% fluid change the first time, 75% the second, 87.5% the third time, etc. Most flushes only claim to get 90-95% of the fluid anyway. I have 180k on the original tranny doing it that way. I got almost 300k out of my GMC that way before I sold it. It still worked.