If the concentration was over 20 ppm it wouldnt take very long, but it would be survivable with immediate medical care. Above that and you might as well not even bother. At 50 - 100 ppm, anyone going in without scuba gear will just add to the total. More than that and they will have no idea it happened because their death will be one or two breaths. As I do not know what the Japanese regulations are (industry standard is alarm and evacuate at 10 ppm).
Reportedly measured at 150 ppm. The odor alone must have been overwhelming.
I think you mean SCBA (self-contained Breathing Apparatus).
SCUBA gear is for underwater use.