Sherlock Holmes first appears in a hospital laboratory, waving a test tube and shouting about a blood reagent, and is introduced to Watson, invalided home from Afghanistan with a shattered shoulder, as a man looking to split the rent on Baker Street. Some weeks later there is a body in an empty house off the Brixton Road: no wound on it, blood that is not the dead man's, the German word for revenge daubed on the wall. Know the shape before you start. Holmes handcuffs his man halfway through, and the book then leaves London for five chapters of Utah revenge, built from the era's anti-Mormon exposés rather than from history. The London chapters go quickly; the Utah ones are where readers stall.
A Study in Scarlet is 43,291 words. At an average adult reading pace of 300 words a minute that is about 2 hr 24 min, or roughly 1 hr 48 min if you read quickly and 3 hr 36 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.