I recently ran a poll on Twitter about the average length of the PhD defense or viva. Even though we all have heard the horror stories of defenses lasting 6 hours or more, the average defense seems to last between 1 and 2 hours.
Here are the results:
How long did your oral PhD defense / viva last?
— Dr Eva Lantsoght (@evalantsoght) January 31, 2018
You can find the wake of this poll here: