Too many to count, when I was a bored student I'd get a game and just play and play and play. I think my longest ever session was 72 straight hours on Civilisation II, interrupted only by juice and toilet breaks. Regular 18 hour sessions of EVE, CS, BF2, Homeworld, Rome: Total War, Galactic Civilisations II gave me a distinctly pale complexion.
Strategy games were always the killers; I've lost months to them. You know you've got it bad when you wake up with a whole new set of ideas on supply-line organisation, army rosters or fleet dynamics.