The is no accurate answer to that question.  The mileage or time of use before major problems occur will vary greatly, and therefore, I don't see a correlation between mileage and expected transmission failure.  It is not unusual that the first few years after a newly designed transmission hits the road, that early failures occur.  But, in later years with updates to the original design, the units become more reliable.

The three major factors in the life expectancy are periodic maintenance, maintaining proper fluid level, and driving habits.