Putting on my hat as an attorney with a working knowledge of United States copyright law . . .
The normal rule of American law is that the copyright holder must file suit within three years of the infringement occurring (the statute of limitations). Depending on which Circuit you are in, a failure to enforce before the expiration of the three years, may or may not bar a claim. Some Circuits have held it will not. Others have held that it may (under the doctrine of laches) if the copyright holder fails to enforce his rights and the infringer and/or innocent third parties suffer undue prejudice as a result. See, e.g., Chirco v. Crosswinds, 474 F.3d 227 (6th Cir. 2007) (a recent discussion of the application of laches in a copyright case). The use of laches to bar a copyright holder's claim before the expiration of the three years is fairly rare.