"The relevant statutes are: 35 U.S.C. §154(a)(1) ("Every patent shall contain...a grant to the patentee, his heirs or assigns, of the right to exclude others from making, using, offering for sale, or selling the invention..."); and 35 U.S.C. §154(a)(2) ("...such grant shall be for a term beginning on the date on which the patent issues and ending 20 years from the date on which the application for the patent was filed...").

If the patent owner has not informed the potential licensee of their possible infringment, once that 20 year date is reached the infringement and licensing capabilites are ended...IMO

Further...

In applying the actual notice provision of 35 U.S.C. § 287(a), the patent owner must give specific and actual notice to the accused infringer of the potential infringement of the patent in question. As interpreted by the Federal Circuit in Amsted Industries Inc. v. Buckeye Steel Castings Co., 24 F.3d 178, 187 (Fed. Cir. 1993), "[a]ctual notice requires the affirmative communication of a specific charge of infringement by a specific accused product or device" (emphasis added). The subjective knowledge of the accused or potential infringer of the patent in question is irrelevant to the determination of whether actual notice has been provided. Lans v. Digital Equipment Corp., 252 F.3d 1520, 1523 (Fed. Cir. 2001) ("Any knowledge the defendants might have had of the infringements before [the patentee] gave them actual notice is irrelevant."). Thus, even if an accused infringer is actually aware of the patent, actual notice must still be provided in order for damages to begin accruing. If actual notice is not provided, damages do not begin to accumulate until after an action for infringement has been filed.