There's no hard and fast rule, but generally anywhere between 70,000 and 110,000 words for an adult novel. Teen novels come in at around 30-45k (which is about novella length), novels for younger kids are less than that.
These are only averages. Novels can be much longer or shorter than these wordages for any market.
45k is approx 200 pages of a standard, B format paperback.