I need a proverb with ironical sense. To answer a "when" question with the meaning never or nobody knows when. for example, in Russian it will be like - когда рак на горе свистнет. I'm interested in something similar in English.
I think that the idiom when pigs fly is probably the most popular and well-known English expression when it comes to describing the impossibility of something happening in a humorous way. In other words, it's used to describe a situation that is either very unlikely to happen or simply impossible to happen at all. Here's how the Collins English Dictionary defines this phrase:
If you say 'when pigs fly' after someone has said that something might happen, you are emphasizing that you think it is very unlikely.
— When would they be hired again?
— Perhaps, as the saying goes, when pigs fly.