Honestly, it's REALLY hard to quantify. There's too many factors. Me, I've got ten kid's books, a comic, and some other small projects out right now and I make a good living, by my personal standards, right this minute at it. But my personal standards were defined by growing up very poor and by being an artist for many years...and ask me again in five years when the series has finished and sales have tapered off. A lot of this is feast or famine.
Also, I live in a fairly average part of the country, cost of living wise. If I lived in San Francisco or New York City, I'd be poor. If I moved to Upper Peninsula Michigan, where property values are about 30% of the rest of the country, I could buy a house with a single royalty check and be comparatively well off, assuming I wasn't bothered by 200+ inches of snow a year.
And I have a spouse with health insurance and a steady job--our finances are separate, but without that health insurance, things would be scarier. But that whole equation, traditionally a very powerful factor in authors keeping day jobs, may be upended when the Affordable Care Act finishes roll out. So what's true this minute may not be true a year from now.
So...where do you want to live? How much ramen are you okay with eating? Do you have someone to share expenses with? Are you trying to put a kid through college? Are you disciplined enough to squirrel away your advance and not go buy a TV as big as a refrigerator? Any expensive medical conditions? Do you have someone who can catch you if you fail?
Perhaps most importantly, how unhappy are you with uncertainty?
Now, me, I'd get to the point where you're getting regular checks sufficient to live on, have contracts lined up, and a solid nest egg, before I decided to take the plunge. (But I hate uncertainty!)
Overall, yes, it's doable, in the right fields, but it's not gonna happen on one book unless you're blindingly talented and blindingly lucky.