There are Dexron life vs temp charts on line. It's a sliding somewhat log scale where it is fine, then quickly degrades as temp exceeds threshold points. When it hits 305F it's pretty much done with zero time constant. The problem is measured average temp vs peak temps that SOME of the molecules are exceeding in key critical hot spot areas of the trans. Those oxidized molecules are then diluted into the fluid capacity. Eventually the fluid becomes overall darker. Hotter areas might be found in the converter, leaving the pump or sandwiched between friction materials during shifts.