1) the work overall. 2) the characters/worlds/whatever the work contains.
I think that the 30 years someone above advocated for works overall is a good length of time. If it hasn't made a profit by then it isn't going to.
But the cgaracters or other intellectual works within the copyrighted work can have a much longer life. Think mickey mouse. The little blighter has been going on for decades through indidual works. One might make profits from mickey for at least ninety-nine years.
So give the intelectual property within a work 99 years. Give the work overall 30 years.
Yes, cutting the dividing line between those two could be quite tricky. A very well written law would be needed. That's a good reason why someone outside of congress/parliament is needed to write the thing...
"You can't get very far in this world without your dossier being there first." -- Arthur Miller