Once I have created something original, do I enjoy the copyright in that work for eternity?

No. Copyright has a fixed term after which the work comes into the public domain, i.e., the content becomes free to be used by others without seeking permission. The term of copyright is as follows:

1. Literary, dramatic, musical and artistic work: Life of the author + 60 years;

2. Cinematographic Films and Sound Recordings: Year of publication + 60 years.

This term, however, could differ in different countries where work has been created.