You've got a great story idea. You believe in this work. Not only do you think it will get you a book contract but it could sell millions of copies. Now if only you had the time to sit down and write it!
Composing a novel isn't easy. It takes diligence. It takes time. It takes patience. It's taken me more than a year to finish one novel. One, I was able to spit out in less than 4 months. But I will say that at no point in time was I tempted to quit my full-time job in order to finish either (even when it meant having only two hours a day to work on my novel). I'm married and have a mortgage. I have not only an obligation to my little family (cat included) but to myself. I could never make a good starving artist. LOL
In a recent op-ed, the executive editor of the New York Times discourages writers from giving up their careers to write the novel. Some have said he's discouraging people from writing novels entirely. That's not what I got. In it, he gives very good advice: Don't give up the practical in pursuit of a dream. Even if you believe in your book, you may not be able to sell it. Even if you have a publisher who's agreed to give you a contract for the book, you may not get the sales their expecting and you might have to return your advance. My parents has always taught me that having a dream is fine... but having a back-up plan is even better. So in other words... don't quit your day job. It's easier to write when you know you can afford to pay your electric bill in a few months.