Basil Hallward paints Dorian Gray's portrait and begs his friend Lord Henry Wotton not to influence the boy. Lord Henry does nothing else for the rest of the novel. Dorian wishes the picture would take his age for him, and it does; over the next eighteen years he wrecks reputations, and almost all of it reaches you as drawing-room rumour rather than as scenes. The canvas keeps the account upstairs, in the old schoolroom that the grandfather who hated him had built for him — and that is also the one room where the book stops reporting and shows you. One chapter is a catalogue of jewels, embroideries and musical instruments with no scene in it at all.
The Picture of Dorian Gray is 78,728 words. At an average adult reading pace of 300 words a minute that is about 4 hr 22 min, or roughly 3 hr 17 min if you read quickly and 6 hr 34 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.