It's seventy-five years and although techincally you don't have to register anywhere to have copyright, having it and proving it are two diferent things.
When it comes to proving it there are only two legitimate routes
1) Store a copy with your solicitor, who prepares a witnessed statement that the document came into his trust on a particular date.
2) Register document with Library of Congress (covers any international copyright issues)
Personally I do both.
Finally, if you've invested years into a project and you believe it has real value you hire a good media lawyer, with experience in the field that you're working and get them to protect your asset. (This is going to cost you £250 an hour) But the truth is, once you start getting into serious business then you need proper advice and legal protection and to try to do business and negotiate contracts without is asking for problems.