Quick answer to how long does a copyright last: A lifetime plus 70 years.
The general rule according to the United States copyright office for works created after January 1, 1978 is that the copyright will protect for the lifetime of the author plus an additional 70 years. There are exceptions to the rule and a different set of criteria for copyrights prior to 1978. A copyright for a pseudonymous work, anonymous creation or a work made for hire protects the material for the length of 120 years from its inception or 95 years from its original publication, whichever occurs first. Once your work is copyrighted (following the post 1978 laws) there is no need to renew or reapply. How long the copyright last will be the duration of the author’s life and a supplementary seventy years.