While I do understand and agree that the initial stages of a startup are about Product Discovery, what I would like to understand more is when startups realize

While I do understand and agree that the initial stages of a startup are about Product Discovery, what I would like to understand more is when startups realize that they have hit the right product. Like what metrics can be considered as an indicator that your product is finally converging towards a point where a signinficant chunk of your target audience will find it useful and use it