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

Except as otherwise provided by law, no recovery shall be had for any infringement committed more 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.