I generally finish most linear games easily, but when I'm offered a free to roam world with a multitude of side quests and dungeons... I simply get overwhelmed! I want to do everything! Kingdom of Amalur, for instance, took me almost 45+ hours to get past the first "continent" or zone or whatever it's called. I just get so sidetracked it's stupid! I spend half my time crafting gear and the other half doing every quest I come across and I'm fiercely overlevelled and overgeared by this point haha! Same goes with Elder Scrolls games, especially Oblivion. It was really hard to stick to the main quest line in Skyrim, but I managed to do it, because I knew how much time I spent already on Oblivion.
The biggest timesink of all, though I think is Arcanum: of Steamworks and Magick Obscure. It's an old PC game by Troika, the guys who made Vampire The Masquerade Bloodlines and technically Fallout. I've probably spent more than a dozen weeks in play time in that game.