Dorothy is a girl from Kansas and a kitchen slave to the Wicked Witch of the West. The Scarecrow who wants brains is a bundle of clothes in a treetop. The Tin Woodman who wants a heart is on sharp rocks, too dented to groan. The Witch dares not strike the child, and covets her silver shoes, and Dorothy knows the reason for neither. While the Witch sleeps, Dorothy carries food to the Cowardly Lion behind his iron fence and lies with her head against his mane.
The Wonderful Wizard of Oz is the story of Dorothy and her dog Toto, blown out of Kansas into a country that still has witches, and of three companions she gathers on the road to a wizard nobody has seen, each certain something was left out of him. It is brisk and matter-of-fact about marvels, and stubborn about one homely idea, that a gray farm with Aunt Em on it beats the Emerald City.
The Wonderful Wizard of Oz is 39,140 words. At an average adult reading pace of 300 words a minute that is about 2 hr 10 min, or roughly 1 hr 38 min if you read quickly and 3 hr 16 min at a more deliberate 200 words a minute.
That count is measured from the complete public-domain text rather than estimated from a page count, so you can compare all the books side by side.