A friend and I both got colds in high school (boarding), so we spent 3 days huddled in one of our rooms, playing Day of The Tentacle, and a one of the LucasArts titles. Except for (not enough) sleep, that's all we did for 3 days. Even being sick as a dog, it's one of my happiest memories. I love games, and I do love single player games, but my happiest memories of games are playing in the same room, with friends.
I also played RE4 when it first came out, with a very good friend at the time. I don't know how many hours exactly, but it was a lot, over many days. We unlocked EVERYTHING, absolutely everything.