My first novel took about 3 years. However, I must add that I wrote the first 50,000 or so words longhand. Then had to type that out when I got a computer, then pick up where I left off. (though finished and not bad, it is still under the bed.)
Second novel took about 5 months. (edits to start next month)
20,000 words into the third, which took about 3 months, but I haven't picked it up in a long long while.