Wow, you've certainly pinpointed the right question. However, for better or worse, you have to answer it for yourself. There is no magic formula that they teach in business school or pass from generation to generation--it's all a matter of who you are, how much risk can you take, and how much uncertainty you can deal with.
You have to deal with your gut.
Also, consider your situation. How deeply are you in debt? How much do you and anybody else depend on your income? Do you have a strong family that can serve as a safety net? If you're young and unattached you can probably take more risk than if you're already on the edge of debt and supporting others.
If it's any consolation to you, you have a lot of company.
Question added to topic Starting a Business • January 18, 2008
At what point should I make the full leap from my day job to my startup?
I have a photography business that I'm starting up, but don't know at what point I should make the full leap from my day job to this startup. Is there a certain amount of income I should reserve before I make the transition?