Everyone writes at different paces. Stephen King says if it takes longer than 3 months, you should probably stop and move on to something else (but he doesn't have a day job that isnt being a writer!)
It took Charles Fraszer 10 years to write Cold Mountain and Juniot Diaz says he's a 1 book every 10 years kind of writer.
For me, who has a day job, it took me 3 years to write a historical fiction novel on a post civil war theme. I did a lot of research and went through many many revisions of my novel before I felt it was completed enough to being to submit to agents and publishers. I will still be open to changes and addtions if others have suggestions but it was a three year process from the conception of the idea to felling like "i finished a novel".