How Long Should You Stay in a Job?

I am puzzled on some of my colleagues who refuse to look for another job when all signs indicate they should move on such as happiness, motivation, progression, salary, etc. They see a lot of negative signs and no where near promotion or a great career ahead, but still they stay longer than I could understand.

I just read this article from Yahoo about knowing when to quit your job. In an excerpt:
So how much should you tolerate? People often stay too long, says Utah-based consultant and trainer Sherron Bienvenu, professor emerita at the Goizueta Business School at Emory University and visiting professor at the international M.B.A. program of the Helsinki School of Economics. They stay because they like the location, they have a close friend at work, they don’t want to let their co-workers or subordinates down, or, simply put, they don’t want to lose the cash and benefits.
How long do you plan to stay?
[tags]Career[/tags]