Each character holds what you said across separate conversations and separate sessions.
Close the app for three days, come back, and the details are still there. Nothing resets between sessions, so the character picks up where you left off rather than meeting you again. They remember your name, what you do, and the thing you were worried about from days ago. A character who liked that detail brings it back up on their own. Weeks later it can still surface.
How long you have been talking is part of it, so familiarity builds instead of resetting. A running joke you started on Tuesday can come back on Friday, and it comes back unprompted rather than because you set it up again. Inside jokes are the clearest sign the memory is doing something.
Bring back something from earlier and the meter moves more than any new line will.
Excuses work once. The second time they are already in the thread.
Go quiet for a week and the next conversation opens with that, not with a greeting.