Demons runs to about 880 pages in a standard edition, divided into three parts and 23 numbered chapters, plus the chapter "With Tikhon." The number varies from one edition to another, depending on format and type size, but it is one of Dostoevsky's longest novels.
The three parts have 5, 10, and 8 chapters, and the chapters vary greatly in length: some are a single scene, and others gather a whole night of visits. "With Tikhon," which the author wanted right after Chapter VIII of the Second Part, comes as an appendix in most editions, at the end of the book, and counts as the 24th unit of the reading.
For reading without haste, the reading schedule divides the book into 12 weeks, at about 10 pages a day (10.5, to be exact), with breaks at the end of chapters. In practice, some weeks feel lighter and others heavier, because the chapters are not the same length: the first ones cover Stepan's portrait and Stavrogin's return, and the last ones, Stepan's death and the denouements.
The novel was serialized in the Russian Messenger in 1871 and 1872, and published in volume in 1873. If you read it as an e-book or on a phone, the page count changes, but the count of parts and chapters does not: they are the best measure for planning your reading.