It all depends on where you live, as the "statute" is obviously set by lawmakers. Best to talk to a lawyer.
For instance in Ontario, Canada, there's a piece of legislation for defamation that has a 6-week notice period for a broadcast:
http://www.weilers.ca/QuickTips/index.asp?ID=168
(yes, I know there's a difference between libel and slander....although in this day and age, things are more blurred, and it's simpler to call it all defamation)