We can’t really talk about the average viva. We could calculate mean lengths, modal questions asked and so on, but it wouldn’t amount to much.
There’s far too much variety in research for us to make claims about a normal viva either. There are norms within disciplines and departments that arise from specific practices, but they have to be explored by candidates in their communities.
The typical viva is knowable though.
It’s typical to have two examiners, one internal and one external. It’s typical for a viva to be between two and three hours in length. It’s typical to expect everyone present to be prepared. The most common outcome for the viva is minor corrections.
Average makes the viva sound predictable (and dull). Normal implies there is a “right” process for all vivas. Typical reminds us that there are common expectations. Neither normal or average, they just are.