If I'm infringing somebody's copyright on an ongoing basis, and the copyright owner knows about this but does nothing, can they still decide at some point to sue me?
This was answered in Petrella v. Metro-Goldwyn-Mayer, Inc. 572 U.S. ____ (2014).
17 U.S.C. §507 establishes a three year statute of limitations. A copyright owner can only recover damages for infringement that occurs within three years of a suit, but delay in filing, no matter how long, cannot prevent such a suit.
Laches cannot be invoked as a bar to Petrella's pursuit of a claim for damages brought within §507(b) 's three-year window.
Section 507(b)'s limitations period, coupled to the separate-accrual rule, allows a copyright owner to defer suit until she can estimate whether litigation is worth the candle.
A defendant can still rely on estoppel, though, if the copyright owner "engages in intentionally misleading representations concerning his abstention from suit...".
And laches can constrain the kinds of relief available. "For example, where owners of a copyrighted architectural design, although aware of an allegedly infringing housing project, delayed suit until the project was substantially constructed and partially occupied, an order mandating destruction of the project would not be tolerable."