If you're an IP attorney, Storm Raven lemme ask:
I am a lawyer. I have handled some trademark issues, and a few copyright matters, but not for a while (I work for the government these days, and copyrights are not so much of an issue for us). But:
Would knowing failure to pursue copyright infringement impact the ability to win a judgment of recovery for that specific infringement? (In other words, if you know I'm pirating movies from your studio and you make no move to stop me for six months -- say, even to the point of an internal memo stating, "Nah, not worth the effort" -- at which point I stop of my own accord, can that knowing failure keep you from getting any recovery from me if you change your mind?)
Probably not. The only legal theory I could think of that might prevent you from recovering in these circumstances are laches, and that isn't normally applicable in cases where a statute of limitations in in place. I know of more than one case in which a plaintiff allowed a defendant to run up a fair amount of potential damages and then stepped in - that didn't prevent the plaintiff from recovering.
If the plaintiff gives the defendant some sort of concrete basis for believing their copying is okay, that might run against them (it would then depend on whether they gave implied consent), but that would require some sort of actual communication between the copyright holder and the infringer, not merely a lack of action by the copyright holder.
What you are trying make out here is analogous to a case for adverse possession - if you live on a piece of property under some sort of colorable claim for a long time without the true owner taking action against you, then you end up owning it. But adverse possession cases are hard to make, the duration needed is usually twenty years or more. The courts simply don't like squatters very much, and don't have a lot of sympathy for copyright infringers.
Disclaimer: I am not offering legal advice to anyone online. Not only am I probably not licensed in the jurisdictions most of you are living in, I do not know any of the specific facts of any case you may be dealing with. Anyone relying upon what I say in this thread to take actions does so at their own risk. The information I am providing here is for general educational purposes, and based upon my own experiences and research. If you have actual legal questions, go and conslut directly with a lawyer in your home jurisdiction.