One thing I haven't seen articulated in the comments here is that there is a big difference between starting a consulting firm and what we call "start-ups".
A consulting firm has very little risk compared to a "start-up". With a consulting firm, you either find clients or you don't. Ideally you should know who your first 2-4 clients are before quitting your job. Probably 6-12 months of living expenses is enough. Taking loans for this is fairly low risk. You won't get equity-based investment from a VC or angel for this.
A start-up is a company that sells a product (though the product could be deployed as a service "subscription"), and it takes time to build that product. For a start-up, you need the funds to make it through development of the product AND selling the product enough to have revenue. A start-up is a much longer process and much higher risk. I would say if you're not doing outside funding, you need 2-3 years of savings plus business costs. If you are doing outside funding, beware that fundraising is VERY distracting and if you don't have good pitching results, it can be very demotivating too.
I have done the start-up scene with and without VC money, through IPO, and through disasters. There's pros and cons to all of it. As other have said, it's imperative that you are building something people will pay for.
What I do is make a datasheet of the product under consideration and then I go out and talk to customers. If they say "this sounds good" it's NOT good enough. They have to say "When can I buy this?? I NEED THIS NOW" for you to know you have something worthwhile. Otherwise, they are just being nice.
And of course, as others have said:
* If you have family, they have to be on board too.
* You have to want it really bad. If you are doing things right, something will go wrong every day. There will be a new problem, a new issue, every single day. Perhaps one a week that could destroy everything. A new competitor, a serious bug, a fundamental flaw, a leaving co-founder, an investment that falls through, the economy with your funds in the market collapsing :-), a lost customer, a disappointed analyst. You have to be able to slog through it.
* Don't do it alone, but also do not bring in other people who do not want it badly. Get a lawyer to set up things correctly and do not let it be the case that a co-founder who leaves gets too much stock.
Good luck!