When is an idea the "right" one for an entrepreneur?

Entrepreneurs get started in many different ways. Some people never desire to become one, but they have an idea that their company won't support so they go out on their own. Some people are just in the right place at the right time. Some people have always wanted to be one. But the question I have is for those who want be entrepreneurs but aren't.

I know a lot of people like this, people who have all the skills and drive, some even have the money, they are just waiting for the idea. But the question I have is, when is an idea good enough? Too long waiting for the "right opportunity" could mean your window passes by. At the same time though, if you just jump on any old idea for the sake of entrepreneurship you risk being tied up in a startup that needs you at a time when a good idea really does come along. I've thought about this a lot because some seemingly good ideas end up failing and other ideas that seem destined to fail end up succeeding. A lot of us are waiting for that great idea that we think will be a home run, but history says no, it probably won't be. Of course, if we don't at least think it will be a home run how can we give it our all?

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I'm torn about what I think. Should entrepreneurs wait for that perfect intersection of passion, knowledge, experience, opportunity, luck and money? Or do you just need two or three of those to hit at the same time? Let's face it, we don't want to end up like Pete Best** and be busy with something else when we could be having our moment in the sun. But at the same time we don't want to reach retirement age while never having fulfilled our dream of giving the entrepreneurial route a shot. What's a would-be entrepreneur to do? When is the situation good enough to make that leap?

**Pete Best was the Beatles original drummer who was fired before they made it big.