Notes from Underground has about 140 pages in an ordinary edition, divided into two parts: "The Underground," with 11 chapters, and "Apropos of Wet Snow," with 10. The number varies from one edition to another, depending on the format and the type size, but it is one of Dostoevsky's shortest books.
The division into chapters is misleading about length: Part Two has fewer chapters, yet it is more than twice the length of Part One. Part One is a monologue of shorter chapters, and Part Two gathers the three long episodes of the narrator's youth.
Reading about 10 pages a day, the book can be finished in about two weeks. A natural division is to read Part One over a few days, without hurry, since it is the densest, and Part Two at a looser pace, one episode at a time: the officer and the collision, the dinner, and Liza's house.
Depending on the edition, the book also appears under the title Notes from the Underground. If you read on an e-book or a phone, the page count changes, but the number of parts and chapters does not, and those are the best yardstick for planning your reading.