Patents can be invalidated by the patent office after six years of continuous use by a person or entity, in most cases, as determined by a panel of patent examiners.
This panel includes three patent examinators, and the patent examiner must agree with the panel.
The patent examiner can, for example, revoke a patent after six months if there is a change of ownership.
The panel can also declare a patent valid if there has been more than one person or business who has used it, or if the patent is valid for more than five years.
The judge can then order that the patent be cancelled or invalidated.
For more information, see: http://www.uspto.gov/policies/patents/en/index.html