Even though I have not taken the leap yet, but working on it with a solid plan. Overall, this is a combibation of subjective and objective items. Listing some of mine below which could give you an idea even though YMMV:
SAVE SAVE SAVE. Did I say that enough ? To your question about 6-12 months rule, it is the standard advice based on assumption that it could take you that much time to get back on your feet if you fail etc. For example, I will probably feel comfortable if I had 100K stashed away safe. I dont care if thats 6 or 12 months.
This is very subjective and could really vary. Are you married or single ? Do you support a wife/kids or single ? Obviously, if you are single, the risk appetite could be higher but if you have a family to support, you might want to get things in order. For example, do you have health insurance thourhg employer and have kids ? Can you switch to your wife's if possible ? etc. etc.
I've been working for a long time, so having savings already was the first step.
Like codegeek said, family situation and health insurance are important, luckily everyone is in support of me doing my startup.
A consideration that was one of the last things to solidify the decision in my mind was people telling me I could get another job easily if things didn't work out. A couple of them were actively trying to recruit/refer me, so I knew I would be in good shape.
A fellow entrepreneur told me, you better be hell bent on what you're doing, because doing a startup is hard. For me I'm obsessed about trying this startup, so I knew it was only a matter of time before I had to leave and give it a shot.
So no precise metrics. Just a gut feeling of: I can't take another day/week/month/year going by without giving this a shot.
I worked as a consultant for 11 Months right out of college. During that time I launched a side project that got some traction and a little press but didn't really go anywhere. I put it on my resume and started applying to startups in the Bay Area. The project got my foot in the door for some interviews and I was eventually hired as a lead dev for a YC company.
I didn't create a startup but I was able to work in my spare time on a project that proved I could ship a product, which enabled me to leave my soul sucking corporate consulting gig.
I planned my departure in my head for a long long time but never got any traction with side-projects - which is understandable in hindsight. Then one day I got sat down with the boss and tried to hide the grin from my face as they explained the redundancy package I was being offered (7 months salary).
So, it was planned but I was still pushed in the end - and it was great.
I'm also interested (not in corporate America, but in corporate anyway :) ).
Me, I'm thinking of getting some freelance jobs first (without quitting) or other source or side income and then quit. I'm thinking of my car as emergency money, and I know I can get a job in a very short time if I need to come back.