Some of my favorite authors can put out as many as four books a year. That's one every three months. Can it be done? Absolutely. But that also requires dedication and no outside interference. I asked one of those authors what her process was and she said she writes from an outline. She's a plotter. That doesn't mean there isn't pantsing involved (writing by the seat of one's pants), but she has a roadmap that starts at the beginning and takes her to her destination. That's good business sense, and it helps her meet publisher-imposed deadlines.
For me, my best showing was three books in one year. That's one every four months, and I should point out I can write them faster, but making them readable is a whole other process. I'm also more of a pantser, so derailing can cause unnecessary detours that extend the process, but I like the scenic route. Following the outline superhighway takes away some of the fun of discovery, uncovering hidden gems you didn't know were waiting for you. That doesn't mean I don't work with an outline at all. Like taking a road trip, I have a general idea of what direction I'm going. I just don't always take the straightest route.
Then there are authors who only put out one new book each year. But you know what? That's usually one really good book. One author I've read takes more than a year to write a book - in fact, as long as two years. I wouldn't recommend that process. For starters, you risk losing your audience, and in my humble opinion, that particular author lost interest in her characters in the continuing series and it reflected in the subsequent books. Granted, her books were VERY long, so she needed the extra time, but the stories begin to feel like guests who have overstayed their welcome.
Everybody's process is different, and another thing to consider is word count. How long is your book going to be? And then there's the muse. Sometimes my muse is sitting on my shoulder through the entire process, and sometimes I get stuck in the muddle in the middle and struggle to push forward. Let's not forget outside distractions. Like a day job. Or family responsibilities. All of these factor in.
The most important factor, in my mind, is being passionate about what you write. You should love every story you tell. In all honesty, I have a couple of books sitting on the shelf that, after re-reading them, didn't pass muster. If I lost interest in them, certainly my readers would, too. So how long does it take to write a novel? As long as it takes to make them "good."