Are they going to immediately go belly-up or have their bodies melted off until there's nothing left but bones? No. But they can die in under an hour because people who DON'T LISTEN and ALREADY KNOW EVERYTHING don't pay attention to practical experience and simple science.
Ammonia (even in VERY SMALL amounts) can and will kill MANY fish. Just because it hasn't happened to you doesn't mean it doesn't happen. It just means you've gotten lucky. The last thing any of us want to happen is for someone new to keeping to come in here and listen to someone with (obviously) extremely limited experience spew off on a subject on which they're obviously ignorant.
I try to give only good, logical advice. If I don't know, I don't post and I read what the EXPERIENCED have to say. That's the whole point.