Don't quit your day job. It's the oldest piece of advice in the writer's life plan.
And, for most of us, it's a good one. Some writers are incredibly productive, flexible, and tolerant of pain, and can make a living from their writing alone--the best way to figure out if you can handle it is to read Kristine Katherine Rusch's excellent Freelancer's Survival Guide.
Most of us can't. I've talked to successful writers with decades-long careers who carried credit card debt and had no money in their retirement accounts.
What I thought when I heard that was: "Why don't you just get a job?" But I didn't say it, because I didn't want to sound like a parent, or a non-writing friend, or just a clueless buffoon in general. That question has certainly been asked before.
But it's still a legitimate one--and one I actually now have standing to ask.
I write slowly, with undependable quality (I throw out much more than I finish), and then edit endlessly. As a result, my end productivity is low, and the final product eccentric and hard to sell. And, as I know from experience, more free time translates into only marginal increases in my productivity.
So I am systemically unable to earn a living from my writing. It took me a while to face this. After a lot of work, thought, and planning, I have found that a disciplined few hours during the week actually generates stories and novels. Figuring this out took some time.
And during the day I manage marketing programs, write copy, think about customers, and learn more about whatever field I'm currently pushing--right now, sophisticated financial services to investors in multifamily housing. I work with focused, intelligent people who are completely different than me. It's challenging, and, a lot of the time, it's fun. And I've gotten pretty good at it, which has its own satisfaction. It also lets me understand how people do business, something completely alien to me before.
So, don't quit your day job. But make sure your day job isn't a miserable soul-sucking torment.
And make sure it has a good health plan.