When I was younger, I couldn't completely lose track of time since my parents kept an eye on how much time I spent playing. Compared to my friends' parents, mine were pretty cool and let me play a lot but still. The games I played the most then were Warcraft 2, Baldur's Gate and Starcraft. Time would go by fast but sooner or later I had to stop playing.
I think the first time I really lost track of time was in Warcraft 3. I faintly remember sitting down one Friday afternoon and played game after game with a friend. Before I knew it, it was 1 AM.
Then there was KotOR. I remember it came out on a Thursday and I was pissed because I had a chemistry exam the next day and I couldn't afford to play. Come Friday evening, I played it until Saturday morning. Star Wars + BioWare was a wet dream come true and the world stopped spinning when I fired it up.