I can usually withstand about 45 minutes of boring stuff before my mind starts to wander around and starts going on an imagination joyride. That 45 minutes can be anything from just waiting around, being stuck in a boring conversation I have no interest in or know nothing about, and anything along those lines.

Because of this, often I start losing attention in class during the later half and start watching the clock religiously like it's New Years Eve and I just can't wait to bail out of there. Unless the class was really interesting or seized my attention somehow, my body was sitting there in class but my mind was basically many millions of miles away.