don't hold your breath
Definition from Wiktionary, the free dictionary
Suggesting that if one were to hold one's breath while waiting for the expected event, one would die before it happened
- (idiomatic, imperative) "Don't wait." Said cynically to suggest that what has just been mentioned to is unlikely to happen soon or at all.
- The government says it's going to introduce free meals for all schoolchildren. — Huh, don't hold your breath.