Aw heck, I'll buck the trend give a concrete answer. It takes anywhere from several hundred to several thousand hours to write a novel.
James Maxey -- an accomplished author whose skill with words I slaveringly admire --
recently drafted an entire novel in a week. Not a task I aspire to emulate, because he did it by cramming all of that work into a 168-hour period. He'll probably spend at least as many hours revising before the book is ready for public consumption.
At the other end of the spectrum we have George R. R. Martin, who is by this point infamous for missing his publication deadlines. The fans do not care. He lovingly gives each book as much attention as it requires, and the result is, each time, a masterpiece.