Disclaimer: It's all speculation. In other
word, no one knows what Oracle will do in the future.
That said, I don't believe any
Java actually expires. They might stop offering it (retiring an exam) but once certified on a particular version, no need to renew the same exam, as is required for certification in some other fields. You can, of course, take upgrade exams for new versions of Java. This is good for your education, though, as you will learn new features of the language.
As an extreme take Java 1.5, which came out 12 or so years ago... the certification exam is only being retired next month! And 6 and 7 are still quite active. Furthermore, Java 9's release date was pushed out again. In other words, 8 will be likely good for awhile.