How long does it take for pilots to become unconscious after an uncontrolled depressurization at FL 340?
According to: Time of useful consciousness:


  
  Altitude (measured barometrically) - TUC
  FL150 (15,000 ft; 4,550 m) - 30 minutes or more
  FL180 (18,000 ft; 5,500 m) - 20 to 30 minutes
  FL220 (22,000 ft; 6,700 m) - 5-10 minutes
  FL250 (25,000 ft; 7,600 m) - 3 to 6 minutes
  FL280 (28,000 ft; 8,550 m) - 2.5 to 3 minutes
  FL300 (30,000 ft; 9,150 m) - 1 to 3 minutes
  FL350 (35,000 ft; 10,650 m) - 30 secs to 1 minute
  FL400 (40,000 ft; 12,200 m) - 15 to 20 seconds
  FL430 (43,000 ft; 13,100 m) - 9 to 15 seconds
  FL500 (50,000 ft; 15,250 m) - 6 to 9 seconds