No one should be lecturing for an hour and a half. If that’s happened to you, I’m sorry. That’s rough. I’ve sat through a lot of lectures myself over the years and I too get antsy after the first 10–20 minutes. If a class was scheduled for 4 hours, I would expect there to be a break every hour or hour and a half for 10–15 minutes, depending on how frequent the breaks are. Or, that you could take a break whenever you need to because it’s more of a free work period after the teacher does some introduction or something.