Когда рак на горе свистнет I guess is an equivalent to "when pigs fly" saying that something is very unlikely.

"don't hold your breath" also means that something is extremely unlikely, however while also imploring a person not to have undue reliance on it occurring.

- Is your company hiring?
- I'll ask the manager if there are any places available, but sales are down this year, so don't hold your breath.

I don't know of any idiomatic Russian expression that would fit here - if I was trying to say something similar, I would say я бы не рассчитывал на это. Maybe someone else can come up with a suitable idiom.
достаточно хорошо

И да, вы правы, что это также значит "маловероятно случаться".

Спасибо!