Say up front what would make you stop, and record here what actually happened.
Superhuman published their own progression, which is unusually honest for this genre because it starts with a failing number:
"When we started this journey in the summer of 2017, our product-market fit score was 22%. After segmenting to focus on the very disappointed set of users, we were at 33%. Within just three quarters of our work to improve the product, the score nearly doubled to 58%."
Two things in that sequence are worth separating, because they are different kinds of move and conflating them is how people misread the case. 22% → 33% came from narrowing the definition of the user, not from shipping anything — the same product, judged by a better-chosen population. 33% → 58% came from three quarters of building. Only the second is product work; the first is a decision about who you are for, and it was the cheaper of the two by an enormous margin.
So the honest endings for a discovery programme are: the hypothesis held, build; the hypothesis failed, and here is the specific replacement; or — the one most often missed, and the one Superhuman's first step actually is — the segment is not a segment, so narrow it before interviewing anyone else.
What does not belong here is "we did thirty interviews and they were all quite positive." That is a description of an activity, and it is what a programme produces when nobody ever went looking for the no.