Keep it for reference, try to sell it to another bookstore or donate it to us. This depends on the book and the publishers. Textbooks, like those in Math, Science, Economics, and Psychology come out in new editions every 2-3 years. Other non-text type books, like those you might find in a general bookstore might never come out in a new edition. These things are unpredictable.
Many factors determine how long an edition lasts until it is updated.
Sometimes publishers make new editions to help eliminate used books. This is becoming a larger factor in determining the life of an edition. Publishers and authors do not get royalties from the sale of used books. New editions also come out sooner now due to the rapid growth of information. Of course this isn't true for all subjects or authors.