In point of fact the length of protection of copyright
changes over time. And for a good part of the last century
the period has been extending faster than time goes by.
Should this continue then pro-copyright pressure groups
shall truly circumvent the US Constitution's statement that
copyright shall be for limited terms by making the term of
protection "forever minus a day".
An interesting question for the lawyers therefore arises if
you borrow code examples from this site. Legally the
copyright is clear, you need permission to use it in your
own code. But practically...people wouldn't post if they
were going to object to your using it!