Cash on hand is just one important variable in a startup’s life, but it’s not necessarily the most important. What matters most is the number of iterations the company has left.
What's interesting is on the Web these days often there are actually TWO different phases of iteration. The first phase is where you're looking for product fit. Paul Graham at this stage says "Make something people want."
Google did product fit early on (get users, see massive user adoption and traction). But it took Google years to figure out the business model, which is phase two. So you built something people want... now how are you going to make money doing it? Because until you figure out both phase one and phase two, you're gonna have to go back to the fundraising trough again and again. You can't plant the flag and declare victory until you got it covered.
So when you calculate how many iterations you have left, be sure to leave leave time for phase two.
Hat tip to @manukumar.