don't hold your breath!

Don't hold your breath. {usu said with pessimism or cynicism}
Something that you say in order to tell someone that an event is not likely/is not going to happen soon (be prepared to wait) or that it will take longer for it to happen than you can possibly hold your breath.
~ Don't expect too much (or you will be disappointed) — не раскатывай губу; не особо на это рассчитывай
~ Don't wait with excitement for sth good to happen (or you'll be waiting forever) — И не мечтай! И не надейся! И мечтать нечего. Размечтался тоже. (контекст.)

Examples:

Co-worker 1: My neighbor's company is thinking of moving to a 4-day work week.
Co-worker 2: Really? Do you think our boss will ever consider such a change?
Co-worker 1: Dont hold your breath! (JenniferESL)

...The City will repair the street but don't hold your breath. (WM)
...She said she'd phone but don't hold your breath. (CID)

...My classmate told me my speech was good, and I should expect a high grade. I told her, "Don't hold your breath." Our professor isn't kind when it comes to giving grades. He's very demanding. (JenniferESL)

More examples from McGraw-Hill Dictionary of American Idioms and Phrasal Verbs:

Tom: The front yard is such a mess.
Bob: Bill's supposed to rake the leaves.
Tom: Don't hold your breath. He never does his share of the work.

Sally: Someone said that gasoline prices would go down.
Bob: Oh, yeah? Don't hold your breath.

see also
[мечтать не вредно]
[fat chance]
[dream on]
[in your dreams]
[держи карман шире]
[ни малейшего шанса]

hold one's breath