I agree with the previous answers.
Basically, my "cues" to leave the table are:
*When I feel I don't have any skill advantage on any of the players and I feel that no matter what I do, I'm breaking even or losing in the long run.
*My preferred play style doesn't work at all and is being completely demolished (I know, that your play style should be elastic and you should adapt, but if your actions are nearly always being followed by a nearly perfect counteraction, I kind of stop enjoying the game).
*I have many bad beats in a row.
*The game is generally not fun to play.
*I am tired, hungry, angry, sad or basically in any negative emotional state, that has an effect on my play.
*When I have 2,5x-3x of my buy-in on the table and I don't have a significant edge on any of the players or there are players who seem to have a serious edge on me.