|Writing in bed has a long and honorable pedigree|
Before I ever sat down to actually try to write a full novel, I had written quite a bit. From very early age, I was writing little stories. I recall specifically stories I wrote in grade school about a boy and his friend that was a griffin, and the story of a man who could turn invisible and ended up being killed by the government.
Through the years I wrote different little stories, often little more than a sequence of short scenes and events that I expanded on as I thought about them. I wrote and drew comics, several books worth, and have always made up stories in my head as I try to go to sleep.
When I considered writing a book, I was always intimidated by the work I imagined it would entail. I knew writing took time, from personal experience, and I knew that to have a worthy book you had to go through multiple layers of editing and rewrites. The whole process sounded like less fun and more misery than I cared to go through.
So I put it off for years, decades even. I knew I could write fairly well but the prospect of all that effort didn't seem worth the end result. I know that this is not unique to me, though. Many people who might be willing to write are intimidated by the thought of the time and effort involved. How long does it actually take to write a novel?
Of course the answer to this kind of question is always "it depends." It took JRR Tolkien nearly 20 years between writing The Hobbit to publishing The Return of the King. Gone With The Wind took 10 years to write. Atlas Shrugged is considered Ayn Rand's greatest work, and it took her 6 years to finish.
On the other hand, some authors can knock off a book pretty fast. Authors such as Steven King and Robert Parker could churn out two books a year or more. Isaac Asimov wrote over 500 works, starting in 1934 and ending in 1992; that's just over 8 works a year, on average.
How long it takes to finish a book depends a lot on its size, how much research is involved, your familiarity and comfort with writing, experience, motivation, distractions, and so on. If you struggle with your health, it will take longer to finish a novel. When Lauren Hillenbrand wrote Seabiscuit, it was a struggle for her as she fought her Chronic Fatigue Syndrome. By the end, she was bedridden for months and it took her five years to finish.
If you're very familiar with the content, need little reasearch, and are a comfortable, quick writer or typist, you can finish a novel quite rapidly. It took me about a month of writing to finish each of my first two books (although I can't write long each day). I average just under 100k words a book, which is short for modern fantasy novels which can be several hundred thousand words each. The Lord of the Rings was intended to be one gargantuan book, but the publishers wisely broke it up into a trilogy.
You can reasonably expect to be able to finish a novel in a few months, but should not feel bad if it takes longer. A book that requires a lot of research such as my current work in progress necessarily requires more time to study and learn - while I was doing the main writing there were many days that I did 2 hours of research for every 1 hour of actual writing.