Just exactly how much ammonia/bleach do you have to combine for it to be life threatening?

Hopefully most of us know better. I have to think that there are people out there that don’t, though. I also have to think that beyond those who have no idea that it is dangerous to mix ammonia and bleach, that there are people who wouldn’t bother to read the ingredients on their cleaners before mixing or combining.

At what point does it become truly dangerous?
Is the risk very high with combination cleaners? For example Ajax with bleach combined with Windex (not really sure what you would use that for, except perhaps your bathroom hardware.. but they were the first two examples to come to mind.)

I don’t recall having ever heard of someone actually dying this way. Have you?