Using Dragon Age: Origins, Fallout: New Vegas, Mass Effect and Might & Magic as examples, Rampant Games' Jay Barnson, in one of his blog articles, argues against the notion that people don't finish long games because of their length, but rather because they "are just too frickin' boring in the middle". Here's a sampling:
You guys know I love games, and I love big, meaty games I can sink my teeth into and play for weeks. That's all good. Many games have spectacular beginnings which combine with the novelty of being a (new game) which can sustain play for hours. And many have some pretty cool, interesting endings. But between that explosive beginning and the equally explosive ending, there's this vast wasteland of grueling repetition punctuated by moments of interesting story progression and the very occasional change of gameplay to spice things up. A little bit of a slowness in the middle can be forgiven in a 2-hour movie. But in a 30+ hour game, it's very easy to lose focus and enthusiasm. As a younger, inexperienced gamer, it may be easier to stick with it, but jaded gamers probably grow bored more quickly.
This is exactly what happened to me with Dragon Age: Origins. And Mass Effect, come to think of it. I had a great time in the intro. For DA:O, in particular, I remember thinking, (Okay, this is heavy-handed and a little formulaic, but I don't care. I think I could really love this game.) And then, a few hours later, I found myself loading up the game and trying to remember what I was supposed to be doing, and why I should care.
Many years ago, I attended a game design panel where they mentioned the success of jRPGs. The speakers noted that the Japanese developers had determined that the average length of a play session (at the time. this was in the late 90s in Japan a somewhat different market from today's western market) was about two hours in length. The speakers noted how most jRPGs had sub-quests or story segments that SURPRISE! took about two hours to complete, with their own sub-story and climax / boss / whatever. The upshot was that the games were designed to be consumed and fully enjoyed in smaller chunks. Each play session was a complete experience, which combined formed a larger whole.