How much effort is required to write a book?

G'day folks, M17 high school student here. After I graduate I'll have like half a year worth of free time before starting university (I'm taking some time off just because I can) and I'm thinking of using that time to write a book. Can I just ask how long it'll take? I've scoured the internet for answers and it's all mixed. Some say it's as short as three months, J.K Rowling apparently took 5 years. So what's the general consensus?