Everybody already knows Henry Jekyll and Edward Hyde are the same man, which is not how Stevenson built it. The book is a lawyer's investigation. Gabriel Utterson is unsettled by a will leaving his friend Jekyll's whole estate to a stranger called Hyde, and works at it from the outside: a child trampled at a street corner, an M.P. clubbed to death with a cane, a locked door, a servant who no longer believes the voice behind it is his master's. Jekyll's own account is held back to the final chapter. Knowing the answer costs less than you would think, because the dread is Utterson's. About seventy minutes, one sitting.
Jekyll and Hyde is 25,601 words. At an average adult reading pace of 300 words a minute that is about 1 hr 25 min, or roughly 1 hr 4 min if you read quickly and 2 hr 8 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.