Stan Baldwin Principal at Baldwin-Scott

This question and the underlying issues strike to the very core of an individual's character. In hundreds of conversations and decades of looking for ways through, around or over thorny problems with business founders, at some point most of them have muttered "If I'd known what this would take I never woulda started!" I can't offer a simple answer because my experience suggests there are a large number of ways to from the delighted "Ah Ha!", through a working prototype, to a sustainable business. Past performance is the best indicator of a CEO's or management team's probability of success ... and the past performance need have nothing to do with the project at hand. A capacity for tenacity is most valuable.
But I see another question in your wording: "How do you know when to quit?" This is terribly difficult. The longer you've been working to make your idea a success, the greater your emotional and material investment, the tougher the call. Is it more money after bad? Or is it just one more push to reach the mother lode?
This is where the advisers you collected when you were thinking about starting this journey come in. Self-knowledge doesn't seem to have become anymore prevalent since Socrates was pondering upon the need. So finding folks that will share your enthusiasm but offer candid assessments of your situation and prospects is extremely useful. Long before the day you stare at the wall and wonder if you've been a massive fool, well-chosen advisers will have helped you see a little further down the road every day. With the added "peripheral vision" they provide, hurdles may be removed, sand traps avoided and, when all else fails, a new path selected.
Perseverance and constant assessment seem to work for the most successful.