At a mates house i ended up playing Midnight Club 3 on Xbox for 8 hours straight, while drinking some Carling. By the time i was finished i had the image of the cars and track 'burned' into my eyes. Honestly, i could just sit there staring at a wall and actually 'see' the image of the screen.
However i just realised this is the PC Games section, so i guess about 6 hours on Battlefield Vietnam/Battlefield 2 is probably the most i've done on PC.