Victor Frankenstein tells the whole thing to a ship's captain iced in on his way to the pole, so you learn early that it ended badly. He was seventeen when his parents sent him from Geneva to Ingolstadt, and four years later he built a body out of charnel houses and the dissecting room, alone, in one room under the roof, up a staircase from everybody else in the building. Shelley never says how he starts it breathing; the lightning belongs to the 1931 film. He runs. What he leaves behind teaches itself to speak, reads Paradise Lost, and comes back to demand a wife. A fifth of the book is the creature's own account. Victor narrates the rest at a constant pitch of anguish.
Frankenstein is 75,038 words. At an average adult reading pace of 300 words a minute that is about 4 hr 10 min, or roughly 3 hr 8 min if you read quickly and 6 hr 15 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.