In my experience a good novel takes about 12-16 months to write. When I approach the writing process, I do three months of character development, two months of plotting and then the remainder of time is spent building the manuscript. That being said, the realities of the writing business are such that after the writing process comes focus groups, revisions, and submissions to agents/publishers. Agents are then going to have notes that you have to determine what to do with and publishers are going to have notes for you to address. So it is not unheard of for a novel to take 3 to 5 years from concept to print. It is this long process that often develops great material.
Persistence is key to the process. Most writers have numerous manuscripts sitting in their closets that remain unfinished, and had they remained steadfast, those manuscripts might have gotten released into the world. Writing a novel is all about revealing the diamond buried in the idea. You are constantly carving away what hinders the viewer (your reader) from enjoying the brilliance of the piece. Art needs room to be created and then it needs to breathe, to be revised, and then to breathe, to be massaged and then to breathe. At some point you have to let go of it and let it out into the world. Determining the right point to release on a never-ending path is never easy. That’s why having the support of a team can make such a difference. It helps to not make such decisions in a void.