Once you have done 20-30 interviews and heard a lot of stories, some patterns should emerge. Depending on your selection criteria you may see strong patterns, weak patterns, or no patterns. The number of interviews by itself is not very helpful.

How do you tell when you are done with the interviews?

The first criteria is the repeatability criteria which is as you are doing your interviews, have you uncovered all the stories? This is really simple, your brain starts saying "I've heard this before". When you start seeing some common triggering event, some common existing alternatives, and the same kind of struggles or jobs being done, and so you are able to predict what people are going to say, that's when you know you are done for this round of interviewing. Otherwise, until you keep learning new things and hearing new stories from the interviews, you should continue running discovery interviews.

Distill down your customer interviews into short stories

When customers encountered triggering event(s),

they had an expectation violation (what broke and what’s at stake).

So they started considering some new solutions (consideration set).

And picked new solution because (unique value proposition)

What kept them from switching (Inertia)

What pulled them to switching (Pull)

What was their experience like (Friction)

Where are they now (Next summit)

Have you uncovered a big enough struggle with the existing alternatives?

Look for evidence of struggle in:

• Friction
• Inertia (people that have the push of an event but they haven't bought anything yet, they are still in research mode and haven't pulled the trigger yet, but because they are still doing active research, there is still energy there. When you post-process interviews, this is something you should look for: Is there really motion?)
• Desired outcome

Have you uncovered a problem worth solving?