The 2014 Scribner Hemingway Library edition of the novel is 321 pages, according to Amazon. The book is of average length for a modern novel.
It occurs to me, however, that perhaps this question refers to the temporal time span of the novel. In that case, the first part of the book, set in Paris, starts during an afternoon, shows Jake meeting with Brett that evening, covers the following day and ends with Brett, the night of the second day, telling Jake she will be leaving for Spain, as she thinks it will be easier on them both if they are apart. The first part of the novel therefore spans about a day and half.
In book II, Jake and his friend Bill travel to Spain to fish. They spend about five days fishing and then move to Pamplona for the running of the bulls. In Pamplona, Jake meets up with Brett and in addition to the running of the bulls, sees a bullfight. At the end of the novel, in book III, the fiesta in Pamplona winds down and Jake meets up with Brett in Madrid.
While there is a gap of time between parts one and two, the narrative of the novel, which does not include the break between when Brett leaves Paris and Jake decides to head for Spain, spans roughly two weeks, with most of Jake's time in the novel spent in Spain.
The idea for The Sun Also Rises came to Hemingway on a trip to Spain in July of 1925. During the trip, he and some friends attended a bullfight and interacted with a bullfighter. He began writing the short story right away and before the end of July of the same year, he had a first draft. Over the next year he added to the story but also cut some of it out right before publication at the advise of F. Scott Fitzgerald. During this time he also changed the title and settings.
The novel as we know it was published in October of 1926 with a total of 19 chapters and a little over 250 pages. It is not a terribly long novel by today's standards. The book is a fairly quick read and keeps things moving as it follows the somewhat depressing but completely relatable lives of the friends, traveling and try to find themselves.
"The Sun Also Rises," by Ernest Hemingway, is a book about morality, values, and the meaning of life. Because these subjects are heavy, deep, and complex, the book has about 260 pages. Every page is worth the read because this novel is considered one of Hemingway's best.