how can you tell that it's 50% contaminated? It is or it isn't, period.
I just visually compare the fluids, and smell them off.
(*) The new fluid is bluish, and the old one's turned little dark yellow.
(*) The new fluid smells petroleum, and the old one smells smoky.
...
I just guess it's 50% - 60% contaminated. It seems it can still go for another 30,000 miles, but that will be it.
ATF drain-and-fill interval is recommended at every 60,000 miles. I'll do it at every 30,000 miles to keep its contamination at less than 50%...
It's just a matter of choice: engine oil is changed 3-4 times a year, ATF can also be changed once every 2-3 years. It's a cheap insurance.
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