While stationary in the required altitude range, I've never seen a POI suddenly appear. Obviously that's anecdotal evidence, but I'd be surprised if it were a time-based trigger. It'd be enormously unsatisfying if you left your ship hovering there, and a POI suddenly appeared underneath you and it contained the barnacle. If correct, it shouldn't matter whether you play one 24 hour session or 24 1-hour sessions, you'll have an equal chance of finding them.
Do you remember how we used to hunt the UA convoys, don't you?
It's very sad to remember it, but we stayed stationary (that means very slowly cruising) in SC for hours, waiting for the right SSS to spawn in front of us. Rinse and repeat.
I fear and suspect that the mechanics have not changed too much, still.
The only slight difference is that, this time, you need to cruise a little much, and the POIs will spawn all around you, one containing the Barnacle, eventually.
The only real innovation since then, is that now we can have also persistent SS and POIs. That's why I prefer to hunt for them. At least they are somewhere.
I hope to be wrong, this time. But I doubt it.
Sorry to disappoint someone here but I doubt those mechanics are changed. There is nothing that lets me think it's different.