During NaNoWriMo the goal is to write 50,000 words in 30 Days. (November 1 – November 30) National Novel Writing Month
From the FAQ page at NaNoWriMo on why a Novel is defined as 50K words..
Our experiences since 1999 show that 50,000 words is a challenging but achievable goal, even for people with full-time jobs and children. This is about the length of The Great Gatsby.
We don’t use the word “novella” because it doesn’t seem to impress people the way “novel” does.
We define a novel as “a lengthy work of fiction.” Beyond that, we let you decide whether what you’re writing falls under the heading of “novel.” In short: If you believe you’re writing a novel, we believe you’re writing a novel, too.