Good question. I had no idea. Fortunately Google came to my rescue, and after a fairly random wander through a selection of websites and blogs I came to the conclusion that somewhere in the region of 70 to 100 thousand words seemed about right, with fantasy books at the higher end of this ( all the world setting etc.) and teen novels at the lower end. Now I like a good structure so it made sense for me to have thirty chapters, divided into three sections of ten each, with 2500 words per chapter. This gave me 75 thousand words in total. Which seemed like a plan.