You guys are missing the point of the original question. Let me rephrase it to make it clearer so hopefully we can have some appropriate feedback.
There are two elements in play:
1) If you leave a game before X minutes, the game will not count as a win/loss for you though will be recorded as a win/loss for those who stayed and finished (or quit after the X minute mark).
2) If the game ENDS (doesn't matter if there are leavers, conceders, citadel destroyed, anything) before Y minutes, the game shows up in the games list as NA, completely unranked for all participants.
The real question is, what is the value for X and Y.
In my experience you can leave a game around the 3 minute mark and have it not count for you, but the game will still be ranked for the people who stayed (given it lasts longer than Y). Also, for the game to count it seems like it has to pass a 5 minute mark. So 3 for X and 5 for Y. These are my guesses though, and I would love to hear if anybody knows the absolute correct values.
Again - this is not a discussion on leaving vs. conceding vs. AI, etc. It is simply how long can you stay before the game doesn't count for you, and how long can the game last before it becomes a ranked game.