Guess it depends on the job. For engineers, you often give a minimum 2 week notice, if not 4 week, because someone has to learn from you your part of the teams project before you leave. Sometimes 4 weeks notice isn't long enough. For the lab I am in, the two people that left gave a 10 week notice so others could pick up their work. That said, I have a few friends in retail, they just told their boss they were leaving and left. But in their jobs, anyone could learn what needed to be done from any other associate in about a day or two.