Singleplayer games (like Bioshock or Syberia) I usually want to enjoy and take my time and drag em out as long as possible... so i usually play at most two sessions of 90 minutes max a day.
If it's something that you can't really win, usually multiplayer games like Team Fortress 2 or MMOs or Oblivion, I can play for hours and hours on end... sometimes all day with short breaks in between.
