Don't hold your breath

Don't hold your breath - something that you say in order to tell someone that an event is not likely to happen.

Example - "She said she would phone but don't hold your breath for that."

Image - https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgxjZCmngd8S_SBb1PB3xQI9jUK8peIIvpEIBtyDX9fGdENGbpbhAfyTUoSVPvfLa813mwscp9xzI7lRj2PrHfaK5dxwnXp6LpT9dVhRyBU77sv38PLACDxJZCmKEoHfyVe_rxJ2RDdi4c/s1600/Don%2527t+hold+your+breath+-+Expression.gif