There's no such requirement for copyright.

There is if you want to keep the copyright. If someone copies your copyright and you knowingly do nothing about it, you'll lose it.

You're mistaken. There is no certainly such "abandonment" clause under international copyright law (which binds most countries), and I doubt any individual country as such a law either.

There are countless examples where copyright has only been defended in specific cases long after the copyright infringement was known of. Notable examples include the song "Happy Birthday" and the Amiga Kickstart ROM.

Feel free to provide any evidence for your opinion if you believe otherwise though.

You certainly don't lose the copyright, but you may lose the ability to sue in situations similar to the one you ignored - because by not suing you gave implied consent to what was going on (at least in the US and I think Britain).

http://en.wikipedia.org/wiki/Estoppel

EDIT: But IANAL, and on reflection I'm not sure I'm not confusing copyright and trademark here.

Hmmm, thought I was right, but I suppose not - thanks. What about trademarks - you have to defend those, right?

Yes