does Copyright on source files expire ?

[Luigi Rizzo]

Someone just asked me permission to move to a 3-clause BSD
copyright some piece of software that I haven't touched in 10+ years.

I said yes, but then I was wondering what happens if the
person listed is not responding or not reachable anymore:
does copyright on source code expire, and if so, when ?
(I suppose it is related to either the date listed on the copyright,
or to the date of some remarkable event for the author).

	cheers
	luigi