At the patch set (fourth position) level, there *is* a time period during which the patch set is eligible for fixes and patch set *updates* (fifth position!) - it's usually one year from the release of the next patch set. After that Oracle support can say "too bad, you'll have to upgrade" for a bug, and will stop producing PSUs for the patch set. But that's not *directly* linked to the Premier Support timeline.
It's definitely complex.
I had to figure it out a couple of years ago and wrote a blog post to refer back to. I think all the policies in there are still in force and the links to Oracle sources are good: http://www.cubecoder.com/making-sense-of-versioning-and-support-policy/