Patent damages may be limited in a variety of ways. 35 U.S.C. § 286 provides for a “running” period limiting recovery of damages to no more than six years before commencement of a cause of action for infringement.

35 U.S.C. § 286 Time limitation on damages

Except as otherwise provided by law, no recovery shall be had for any infringement committedmore than six years prior to the filing of the complaint or counterclaim for infringement in the action.

This is not a time limit on bringing the infringement suit; it is a time limit on recovering damages.The public policy idea behind this limitation is to discourage patent holders from sitting on their rights while damages accrue.

If a patent is infringed while it is enforceable i.e. before it expires, the patent owner can sue the infringer up to 6 years after the patent expires. This is important in a situation where the patent owner may not become aware of the infringement until after the patent expires