According to the Berne Convention, the international source for copyright law, the term of copyright is the author's lifetime plus 50 years. Many countries have lengthened that, however, and in the USA, UK, and much of Europe, copyright now extends for the author's lifetime plus 70 years. (For more info, see this handy chart of copyright duration in various countries.)

Life-of-copyright sounds scary, but it doesn't have to be. It's a rare book that's so successful that it remains on the market for more than a few years--and traditionally, authors can reclaim their rights once their publishers have taken their books out of production. This process is called reversion. It's governed in a publishing contract by a reversion or termination clause, which describes when and how the publisher will take the book "out of print"--out of production and off the market--and also, since simply taking a book off the market doesn't necessarily guarantee the return of rights, the steps the author must take to get the publisher to relinquish those rights back to him or her.

In a fixed-term contract, a reversion clause may not be needed, unless the fixed term is overly long (for me, "overly" would be anything over seven years)--though publishers often do cover themselves by allowing for discontinuation of publication for various reasons, such as slumping sales.