TKD wrote:

However, there must be a time limit for Oyster journeys as this would be the point
at which the journey is declared "unresolved" if you do not touch-out.

Not necessarily. As you can't make two journeys at once, touching in
again (before any touch out) would surely be the point at which that
was flagged?

That said, you could then get away with a very long journey if you
touched in, passed out through open barriers, then passed through open
barriers to start another journey and touched out at the end of it.
That doesn't go to say that there *has* to be a limit, though, it just
means there would be a slight (and probably rare) anomaly if there
wasn't.

Also unpublished is the time you have to make an out-of-station interchange.

That could (but again possibly introducing an anomaly as above) simply
be triggered by a touch-out followed immediately (with no other travel
in between) by a touch-in at a valid station.

Neil