Her husband John, a doctor, has decided she needs rest. He settles her for the summer in a rented country house, up on the top floor, in the old nursery, forbids her any work, and cannot stand to see her write. She writes anyway, in secret, and the entries fill up with the paper on the walls — first the pattern, then a shape that seems to move behind it, then the bars the pattern turns into by moonlight. Gilman endured the same rest cure herself, prescribed by Weir Mitchell, whom John holds over her as a threat. She never names her narrator. Six thousand words, one sitting, deliberately airless — and nothing supernatural is ever confirmed.
The Yellow Wallpaper is 6,140 words. At an average adult reading pace of 300 words a minute that is about 20 min, or roughly 15 min if you read quickly and 31 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.