I've got two kids, a full time job, but I still love long games, providing they're not stretched out unnecessarily.
I've played over 200 hours of Oblivion for example. When I get addicted to a game I get really addicted to it.
But I find difficulty spikes are the biggest barrier to completing games for me. I'll happily play a game for hours, then come up against an insanely difficult piece of gameplay. If I can't find a solution myself, or one via the internet I tend to abandon the game and come back later. It's not unusual for me to leave a game for months or even a year, then come back to it and beat it.
The second main reason I might not complete a game is that it just doesn't grab me - Far Cry 2 is an example. Played it for a couple of hours, got bored, and have no interest in playing it again.