You've played too much Portal when... You all know the rest of this one haha
You've played too much of any game when you find yourself trying to quick-save before doing anything risky IRL
You've played too much Prince of Persia or Assassin's Creed when you don't touch the ground at all while walking somewhere
You've played too much Solitaire when you notice it's at the top of your start menu :/
You've played too much Amnesia: Dark Descent when you piss a little every time you hear a sound or notice any sudden movement
You've played too much of any MMO when you begin to pretend that household chores are quests
You've played too much Two Worlds if you find yourself considering suicide
You've played too much Conker's Bad Fur Day when you notice you've been using unusually large amounts of British swear words