How deadly is ammonia for the Axolotl?

[corner]

I know that ammonia should be at 0ppm at all times, and my established tank has that on a consistent basis. I've been a bit busy with school lately, and I was not able to test Craigs water as frequently as I used to, and when I came across the chance to do so, the levels were at 2.0ppm. I freaked out, and immediately tubbed him, and did a 75% water change, and let it run for a little while, everything that you should do. Craig is completely fine and is behaving like his normal lazy self.

I'm curious as to how toxic that ammonia is? If this happens again, and the ammonia stays at those atypically high levels, how long until the health of the Axolotl declines to the point of no recovery? Can't bear the thought of losing my little guy

 

[JM29]

I've no detailed data for axolotls precisely but let's suppose the toxicity is the same as for fishes.

If you've tested for total ammonia (NH4+ and NH3 together), then 2 ppm is not so much.
If you've tested for free ammonia (NH3 alone), then 2 ppm is a lot and ... your axolotl should be dead. Since it's not the case, I suppose your test gives total ammonia.

At usual pH values, NH4+ is the dominant form (about 20 to 25 times more than NH3).
NH4+ is not very toxic.