Lean startup viability: how to know when to quit

How do you determine the viability of a lean startup? Or more importantly, how do you know when to quit?

In a VC or even angel-backed startup your viability, assuming you are not profitable, is generally based on one thing…. The ability to raise additional funding. When the money runs out either you raise more funds or you go home.

In a lean startup however, there is no such Darwinian fate. By the very nature of a lean startup and its iterative customer development process, a lean startup can live virtually forever and perhaps never find success. Therefore it falls to the entrepreneur to decide when to cut bait.