The length of time that may be deemed unreasonable has no fixed boundaries, but rather depends on the circumstances of the case, although a presumption of laches arises if the patentee delays bringing suit for more than six years after actual or constructive knowledge of the defendant’s infringing activity. Once a presumption of laches arises, the patentee may offer proof directed to rebutting the laches factors. Such evidence may be directed to showing either that the patentee’s delay was reasonable or that the defendant suffered no prejudice.
Evidentiary prejudice can occur when delay has caused the loss of key witnesses and/or the loss of documentary evidence. Economic prejudice arises when a defendant suffers the loss of monetary investments or incurs damages that likely would have been prevented by earlier suit.
Once the relevant facts are established, the court weighs the equities in order to assess whether laches should apply to bar those damages that accrued prior to suit. In the event that laches is found, recovery is barred for damages incurred before the complaint was filed. It is important to remember that a successful laches defense bars damages only for conduct occurring prior to the filing of the lawsuit.
The moral of the story is that in order to maintain the integrity of the exclusive right granted in the patent, mechanisms must be in place to discover infringement. Likewise, once infringement is discovered enforcement actions need to be taken swiftly, or else the risk of there being prejudice to the defendant rises, which could limit or preclude recovery. Moreover, even if laches does not ultimately provide a defense, inaction on the part of the patent owner chips away at the exclusionary right and allows a competitor to gain a foothold in the patent owner’s marketplace. Any way you look at delay in enforcement, it is costly."