It wasn't based on real time.  If we end a session with the party going back to keep to resupply (which happens outside of the session), you figure how much time that would take in game.  1 day to get to the keep, 1 day to resupply, and 1 day to get back.  So when you start the next session, whether that happens 1 day or two weeks later, 3 days in the game passed, and creatures would react accordingly (doing their own potential reinforcements).
Never did I ever see us match in-game time with real life time between sessions.  We knew the rule (contrary to the video's claim) but thought it was un-fun.  So we ignored it.