Hi, Timo. First of all, thanks for downloading the game and playing it.
Regarding your question, it is a tough one and I admit I found myself in your same situation. I haven't prepared any rule about the end of the turn, precisely because of how difficult it was to determine it rationally.
When I played, I left it to the 'flow' of the game (as you mention) and I let the narrative and the pacing take the lead in agreement with my partner. I encourage you to do it this way.
But if I'd have to rationalize it, I would say that the turn can be considered finished when the 2 players are satisfied with the answer provided.
Anyway, this is a quick project and I have barely playtested it. So if you have any idea or if you find a way to deal with this issue, I'm totally open to suggestions. If I try them and I see that they work, I will include them in future updates giving you credit.