I just finished my first Nanowrimo -- for those of you who haven't tried it, see http://www.nanowrimo.org. It was a blast, and a good learning experience.
What did I learn? That it is possible to write a novel in a month (okay, I finished the 50,000 words, but the novel isn't technically done yet). All you have to do is spend a couple of hours a day at it. You start with an idea, you sit down every day (or almost every day, I did probably 6 out of 7), and the words pour out. You don't have time to wonder if it's any good, which is probably what slows you down normally.
But you're asking, how do you know it's worth all that time? I don't, but the point is, it's not all that much time = worth a shot, not like agonizing for 5 years on weekends and holidays and ruining vacations and aggravating significant others...hmm, you get the idea.
"Immersion in the life of the world, a willingness to be inhabited by and to speak for others, including those beyond the realm of the human, these are the practices not just of the bodhisattva but of the writer." --Jane Hirshfield