Standard episode times exist because it makes time scheduling simpler. Games, not suffering from such a predicament, are free of the burden of attempting to adhere to scheduling standards.
A game exists for a designer to generate an experience for a player, and a game should go on for exactly as long as the designer feels it's necessary to generate that experience. An arbitrary time constraint has no use here unless it actually contributes to that experience.
People will play a game until they cease to enjoy doing so, and time along has no bearing on it. Instead it has to do with a player's experience, and how that experience changes (or doesn't) over time. You might cease to enjoy a game just because you realise you've spent so-and-so hours/days/years on it, but that's still just because of the player's experience of the game - they now experience that game as a waste of time.
Difficulty level is apparently a significant contributing factor to whether the player would actually like to hang around, but there are no doubt a ton more gameplay factors responsible (e.g. whether the game gets repetitive or remains interesting).