Takes about 30 minutes, if I remember correctly, depending on party composition.
A lot less than I expected. Even in RTwP it seemed to drag on forever, but maybe that's because I paused too much, to the point I practically made it TB anyway.
EDIT: In other news, I was splahed in the face with cold, dirty water when I realized that the combo which was straight-up stupid OP in Kingmaker was going to be a lot less useful in Wrath. I am talking about, of course, putting Communal Delay Poison on your Party and then just going to town with Stinking Clouds, spamming it left and right.
Thanks to Heighten Spell, this combo could've easily carried you through the vast majority of Kingmaker, but unfortunately, this is Wrath and Demon enemies are pretty common. And it seems they all have immunity to Poison, which Stinking Cloud is classified as. Cunts.