I think a lot of it depends on the presenter, the audience, and the content.
I did one for an hour workshop that was about twenty slides long. It kept it
really simple because of who the audience was. But I also ran over because
the audience was very interactive with questions.
I've also seen some briefings go through thirty slides in ten minutes (speed
briefing), take 90 minutes to do thirty slides, and others run out of time.
Most often, in the kind of presentations, I'm seeing, it's about five to ten
minutes a slide because of the discussion involved. If they have 60 slides
and two hours, they're very likely to have speed through or skip parts to
finish on time.