Yeah, 'how much is enough' is a hard number to pin down. If you are single you can better than get by on less, but if you're married and you've got 3 kids, you're going to need more.
I feel 'enough' is a salary that allows you to live at the 'standard' of living for the place you are in and also provides you with enough for some savings. Whether you choose to save or spend is really up to you.
In Japan the average English teacher working for a school or company will earn around $30k/year. That's great money for a young/single person, but not enough to support (long-term) a family of 4 or 5, unless your spouse also works. You could definitely do it on $30k/year, but I know it would wear me down living month to month for that long, especially with no job security
In a country with less expensive daily necessities, teachers could reach that goal of standard of living + some savings for less.