Well, you only are supposed to spend 15 minutes on the assignment. But you can literally spend months or years perfecting a novel.
I read through the examples. The Snowflake method sounds good. It is about building your story, much like a snowflake is built in a cloud, layer upon layer.
So, what do you think you can do in 15 minutes of uninterrupted work? A chapter? Perhaps two? What about an hour??
I note on your bio that you've already written a novel. Think back to that time. How did you approach it?
Like the old joke. How do you eat an elephant? Bite by bite.