OK, so here's the real question: how do you know when what you're doing can be called 'endurance' and not foolishness?
You work on your product; initially everything's great, there's a feeling of 'nothing is impossible'. As the months go by without any traction, you begin to have a few doubts. Finally, after you've spent quite a lot of time and got nowhere, you're really pissed: on the one hand, you 'invested' so much of your time into it. It might need 5% or even 0.1% more effort to become successful. But, how do you know when to stop? How do you know that you're not going down a dead end?
I think the premise of the question is wrong. In business, there's never a case where you're 95% or 99.9% of the way to "success". Success is something that develops over time.
Validating your ideas is the most important part of the journey, and learning how to do it in the context of your particular product or market is what will lead to success. It's also something you get better at over time.
So it that sense it's self-fulfilling; with endurance you get better at validating your ideas, and become more successful.
But some common validators for startups are:
- growth (in traffic, user signups, revenue) - this is considered by far the most important in YC
- user engagement & feedback
- user testing
- a/b testing or multi-variate testing
Whole books have been written about this stuff - most notably by Eric Ries and Steve Blank in recent times. It's probably worth your while to read them.
Like all maxims, "by endurance we conquer" is meant only to remind you of some potential solutions. Knowing when to quit and when to stick with it is the hard part.
You work on your product; initially everything's great, there's a feeling of 'nothing is impossible'. As the months go by without any traction, you begin to have a few doubts. Finally, after you've spent quite a lot of time and got nowhere, you're really pissed: on the one hand, you 'invested' so much of your time into it. It might need 5% or even 0.1% more effort to become successful. But, how do you know when to stop? How do you know that you're not going down a dead end?