Never. That seems entirely impossible for me so far in my life.
People talk about a TV show or whatever being "good to fall asleep to." I can't relate at all. I basically could never fall asleep to a TV show. Let alone at a speech, lecture, concert, play, movie, class, service, poetry recitation, meditation circle, hoedown, etc. Doesn't matter how boring a speech it is or how sleep deprived I am. It's just never going to happen. (Hell. I've never neglected to turn the light off before falling asleep. Like, literally never.)
To be fair, I suppose I am excluding times from before my first memories, so I guess as a toddler things might have been different. Also, maybe as I get older, that'll eventually change? Who knows.
I think I'm unusual in the extent to which I'm unlikely to fall asleep in such a situation. It's my lame superpower. Lol.
I have fallen asleep in other circumstances like the cinema, watching tv with my wife, etc.
I haven't completely fallen asleep on a speech but I have been close, when I was in college there was some talks/speeches and I had to battle to keep my eyes open, and I couldn't stop yawning, but won the battle and didn't fall.
my girlfriend-now-wife wasn't amused with my victory and said something like I was yawning too much and I should have showed more respect to this special invited speaker from somewhere I don't remember...