Consistent transmission fluid operating temp of 195 will give a fluid life of 50k, when it'* all new. During a typical pan drop fluid change your only changing out plus or minus 50% so a 25k fluid change is prudent. I live in the flat country of SC and no towing so I haven't added an external cooler on Bonne. You don't have to tow to run the trans temp up. Way back in 07 I had a rental gran prix that had a factory trans temp gauge in the dic. From Seattle (basically sea level) climbing up to 4000' going over Stevens Pass on US Highway 2 the trans temp reached 250 degrees, I only hope the new owner didn't daly with a trans fluid change. One can find fluid life vs. trans temp via google.
2004 Bonne SLE
1971 Chevelle SS