Quote Originally Posted by iCake View Post
Когда рак на горе свистнет means that it's extremely unlikely that something happens, in other words it won't happen ever. So that's not it
Пока жаренный петух не клюнет в жопу means that something won't be done untill something providing right people with an incentive (usually something extremely bad so they have to deal with something immediately ) has happened. So that's not it too

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.