Tenure - How long should you stay in a job for?

There has been a lot of chat on Procurious and in the media about millennials recently. Millennials (like me) tend to stay in their jobs for less time than their baby-booming forbearers. One of my former baby boomer managers told me that most people my age don't have ten years of experience. When I questioned him on this, he politely let me know that we (millennials) actually only have "two years of experience, but you've had two years of experience five times". So with this in mind, I ask you this question... How long should you stay in a job for before before looking for new opportunities??