What's your opinion on the best time to change it? Half the maximum, like the oil?
To be completely transparent, I do not have any fluid analysis data to support any specific interval. I can only say the the published interval is 150k, and there is no transmission fluid interval listed within the “Special Operating Conditions” section. 75k-100k would probably be a reasonable interval for most people’s use cases.
Out of curiosity I checked on Super Duty; under “Special Operating Conditions” (heavy towing/off-road) it recommends T-Case and Axle fluid changes at 50-60k miles instead of the normal 150k, but the transmission fluid interval remains unchanged at 150k.