I'm pretty lucky. I get to quit my day job (in a bookstore, naturally) in about a month. Not that I'm making enough to support myself, but I'm making money, and that means I'm a writer, and so with the husband Adam's help and agreement I can try to make a go of it.
In the many years that I've been working in the bookstore, at least two people have done the same thing: quit their jobs to go write. Both of them were nice people, they had probably dreamed of being writers for a long time, but it didn't work out for either. One of them now works at a different bookstore, and I've no idea at all what happened to the other. The reason I mention this is by way of advice to anyone thinking they might like to be a writer. Because in the difference between them and me is the reason to keep or drop your job.
I've got a contract, a book forthcoming, a bunch of novels under my belt, and more that I'm working on at every moment. And I'm still pretty unsure that I'm doing exactly the right thing by leaving my job, but in the end I'm starting to feel as if I'm wasting so much time at the job that I can't accept working there any more.
Both of the people who left, I think they'd fooled around with writing, and I think they'd decided if they only had more time they could finally, finally write that book they had in their heads. Finally put down all the words that bubbled up in the quiet times of the night, put together those notes and scribbles into a cohesive whole.
That's not how it works, though. If you're not already using your spare time to put down your words, if you're not already done with a novel, or a bunch of stories, and working on the next one or batch, if you haven't found yourself wishing that you had more hours to write but stuck with that nine to five gig, then you should most definitely not quit your day job. Should you be a writer, you'll already be writing all the time. And hence, if you're not already writing all the time--or at least sometimes, whenever you can spare a moment--you're not going to somehow start doing it when all your days stretch empty and shapeless before you.
Start to write while you've got your job. Make it the thing you do, all the time, around your work and your family (and your hobbies, if you can fit them in). Then, only then, if you think maybe you might have a chance and you have money or support and you've gotten good feedback, then quit your job. Maybe.
I'm still not sure. But I'm doing it all the same.