If you write a book, the copyright for that work is protected for a long time, but the work will eventually enter the public domain under US copyright law. Does the same thing apply to pieces of software released under a permissive open source license? Would software released under a BSD, GPL, or MIT license automatically enter the public domain at some point in the future? I know that copyright law changes frequently and retroactively, I'm just curious about how the current laws would apply.
Theoretically, the license itself is not time-limited - once the code has been released under any of the MIT, BSD, GPL or Apache licenses, it is available under that license in perpetuity. If 200 years from now you wanted to distribute the source to version 5.0 of the Linux kernel via the GPL, you could.
However, when the copyright expires (70 years after the death of the author or whatever), everybody gets the ability to distribute the code however they like so they would no longer be required to follow the GPL, any other open source license, or for that matter any proprietary license. Open source licenses rely on copyright law for their effectiveness just as much as proprietary licenses do, and they all cease to be effective when the copyright protection expires.