I think you should think about it in this way: Do you see any branch leading out of your current product? By a branch leading out of your current business, I mean not both of product and market should change at the same time(minor tweaks are obviously ok). If you don't see that at all then it's time to shut down. One thing that I'm over-simplifying is the patience required to make it to product market fit. If you are making big pivots every month then you are basically trying to avoid the trough of sorrow but if your product is not changing at all even after 6 months despite nothing working and everything being tried out on marketing, then you are too late in making a pivot. Here's an interesting article about this by Mattermark founder: http://www.daniellemorrill.fundamine.com/2013/03/zombie-star...

Since you are using some lean startup terminology, what is your data telling you?

How much empathy have you gained around your particular customer and problem? Have you seen any other obvious pain points that you could be passionate about solving?