More than one four hour session spent sitting in a bar with no forward motion in regards to the campaign would kill the game for me. Heck, having lived through that just one session, I should probably say that doing it for one session would put me close to the edge.
I am all about forward motion. I'm okay if it's RP or combat, action sequences or puzzles, but at the end of the night, I want to look at how I spent my time and see forward motion. I want to know that my time spent in prepping for the sessino was not wasted. Lately, I've gone pretty heavy into combat scenarios because of that desire, but truth be told, so long as I see forward motion, the game is good in my book.
This goes both as a DM and as a player. Two consecutive sessions of accomplishing nothing, from either side of the screen, will make me want to stop participating in a game.
The second thing that will get me to stop DMing is if the players develop attitudes and personality conflicts that come out during the game and ruin my enjoyment of the game. If I get one session of that, I give a warning (unless it obviously is one player, whom I then "ask" to leave the game). Fortunately, that's all I've ever had to do. If a second session happened like that, I would stop the game and never return to it. I don't need that in my life, and don't feel that I need to put up with it.
The third and last thing would be Burn-Out. That's why I'm taking some time away from running games and trying to rejuvenate my imagination.
Hope That Helps,
Flynn