It is difficult to pinpoint exactly when you have achieved product/market fit, but I assure you that you will know success when you experience it. If you are so fortunate to achieve the elusive product/market fit, congratulations! You have passed the startup stage and are now entering the transition to a company which will be exploiting that discovery.
It is important to recognize that such a transition is the beginning of a new chapter in the organization's life, which requires setting new priorities and adopting a different mentality. Typically, the primary organizational objective shifts from learning and validation to securing and maintaining market leadership by scaling operations and developing a sustainable and unique competitive advantage. This is a topic that goes beyond the scope of the Startup Lore, but I invite interested readers to check out Steve Blank's excellent blog as well as the many great books on general management such as Strategic Management by Garth Saloner.
In the last chapter of the Startup Lore, I suggest next steps for interested readers.