I think it's fair to say many of us start building things for fun in our spare time and hope it'll go somewhere. The risk is pretty high if you're only going zero or 100 and nothing in between.
As for what level of traction you need, this varies by project. If you need money to live, then you can fully commit yourself once you can start paying bills with it. If money isn't an issue, you may want to find another metric that's meaningful to you.
Seth Godin mentioned an initial group of 1,000 is a good starting point for getting your idea/business out there. If your startup solves a pain or is fun, this shouldn't be an issue.
