How long till a book/music goes into the public domain?
Cynical answer: Number of years since Disney produced Steamboat Willie plus a few years because it keeps on being extended to protect Mickey Mouse.
Official answer from the government: http://www.copyright.gov/help/faq/faq-duration.html#duration
Other countries have different rules.
MPAA – RIAA: You Have No Right To Perpetual Access
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"We reject the view," he writes in a letter to the top legal advisor at the Copyright Office, "that copyright owners and their licensees are required to provide consumers with perpetual access to creative works. No other product or service providers are held to such lofty standards. No one expects computers or other electronics devices to work properly in perpetuity, and there is no reason that any particular mode of distributing copyrighted works should be required to do so."