Ask HN: When do you think to quit an idea?

You shouldn't just have one idea: You should have dozens. The trick then is to pick the one idea that's the best direction to go in. You can make a matrix chart for this which can address everything from the investment required to the potential revenue, and of course have a column for your passion.

I'll also add to that that a business isn't just an idea: It's the team. So you may start with one idea and it may evolve or pivot over time. For example Twitter was a side project and the iPod was a peripheral device (which wasn't even as cool as the Newton first was if you were a geek).

What is the market telling you? The answer to this question always determines whether I quit, continue, or pivot. My market feedback can take on various forms. Sometimes I'll survey people in the relevant field, other times I throw together a quick demo and use this. Either way, you have to get the pulse on the market. Yes, sometimes "build it and they will come" does work, but most of the time it doesn't. Why not query the market and find out for sure.