As long as possible. If you own the rights, then you can sell it, license it, give it away or whatever. A system whereby you have to reassert your rights regularly removes control from you. If it is assumed that the creator of the product owns their own rights, then it is their choice as to what they do. If it is enforced externally by legislation that an author or composers copyright has to be reconfirmed every few years or copyright is removed entirely, then the creator no longer has control. There will be no incentive for those who derive an income from the licensing of the rights of their music.
See this discussion for an interesting idea that we disagree with and the ensuing discussion.
Music Insight 