Look what I found...
I found an interesting blurb on a Government of Canada website about teaching English in Korea...

"Many others, however, eventually reach the point where they feel it is time to leave. With luck, you will realize this before it affects your life too profoundly. It is time to leave when you begin to be negative about the country and its people. When you no longer want to go to work, dislike your students, become irritated with everything and everyone and have angry discussions with others of like mind, it is time to go. "

I think that it would serve well for many countries.