The Idiot has about 890 pages in an ordinary edition, divided into four parts and 50 chapters. The number varies from one edition to another, depending on the format and the size of the type, but it is a big book, longer than Crime and Punishment.
The four parts have 16, 12, 10 and 12 chapters, and the last chapter of Part Four is the Conclusion. The novel has no chapter titles, only numbers, and it was serialized in The Russian Messenger, in 1868 and 1869.
To read without rushing, the reading schedule divides the book into 12 weeks, at about 10 pages a day (10.6, to be exact), with breaks at the end of chapters. In practice, these are weeks of three to six chapters: the first ones cover Myshkin's arrival and Nastasya's birthday party, and the last ones cover the night in Rogozhin's house and the Conclusion.
If you read on an e-book or a phone, the page count changes, but the count of chapters and parts does not: they are the best measure for planning your reading. Anyone who wants a faster pace can read Part One in two or three days, because it concentrates the whole book's problem into a single day of the story.