Never. Than can extend an investigation by years when new information is gathered, so theoretically they can extend for as long as they want.
Really? The statute of limitations exists to require the government to bring a case within the time frame of the statute. There is no exception for “ continued investigation”.very few exceptions exist for not bringing within the statute. In state court leaving the state is an exception for example.
If you dont file within the time you are out of court. That is the whole purpose of the law.