Yeah, it's not a question they really have answered anywhere. Not surprising.
On the iPhone app, this is all I can find for rules pertaining to it:
DELETE AND FORFEIT:
You can delete a game when its progress is below three turns. Once three turns have been taken, the Delete button becomes a Forfeit button, as you will register a loss for leaving the game past three turns.
NUDGE AND FORCE FORFEIT:
A player that has been inactive in a game for too long can be nudged. If a player has been nudged once but continues to remain inactive, the Nudge button in game scene will become a Force Forfeit button. When used, it will give that player twenty-four hours to make a move or they will be forced to forfeit the game.
TIME LIMIT:
In the Options menu, you can set the maximum amount of time allowed between each turn to 24, 48 or 72 hours for online and Facebook games. If the active player fails to play his turn before the time limit expires, the other players will be given the option to alert this player by pressing the Nudge button.
There isn't anything that states that a game will automatically be forfeited by the person not playing, as Words with Friends does.
I've had some games end in automatic forfeits (I believe), but I haven't paid that much attention to how long or when it happens. I can find these in my Complete Games section, with me stated as the winner. I do notice that some of my games that I did not nudge are no longer on my iPhone, but can be found on Facebook (which I never get on to play). Some are as old as 67 days, but that's because I never nudged. Soon as I nudged, they showed back up on my iPhone. Now to see how long it takes to have them automatically forfeited. I wish there was an automatic nudge option.