In my years advising businesses, from start-up to global giants, the passion that makes them successful is the passion to solve their customer's problems.
So the short answer about when to pivot is to do so when you realize that the value proposition you had doesn't fit the problem or customer that you had in mind, but there is some other problem that exists with those customers, or some other customer who has your original problem. Then pivot to focus on that value prop and business model.
If the problem doesn't exist, or is not large enough to fund your business model, then it's likely not a pivot. You may chose to blow up your business model and start on something new.
Hopefully that will help you understand when you pivot or when you pass.
Let me know if we can help you in anyway, as it all about the customer