I used to be one of those people that would stay regardless of how bad a show was, always clinging to hope that I will find a bright spot to keep me engaged.
That all changed when I fell asleep during Memphis because I was trying to stick it out. I still don't understand how I could sleep through the last half of act one all the way up to the finale when the cast stormed toward the edge of the stage. I'm not a quiet sleeper, but I guess no one was bothered enough to nudge me.
From that moment on, I made a vow to leave when I just can't stand it anymore. Since then, I haven't ever needed to walk out from a Broadway show, but the Guthrie in Minneapolis has done tons of garbage that I have left early from. Tony Kushner's Intelligent Guide... (the world debut) was so horrible, I couldn't even make it to an appropriate break in the performance to exit. I was so excited, but then showed up to see masters of their craft flinging unintelligble text by Tony all over the place with little movement, direction, or context to help the audience figure out what was going on. I don't think anyone, including Mr. Kushner knew what was going on (and he basically admitted that during his pre-show address to the audience). I should have known it was going to be a bad evening when he invited "anybody left at the end of the night" to go out for breakfast (Tony's treat, of course) to discuss the play. A pre-apology is never a good sign.