how long is a piece of string

/hˌaʊ lˈɒŋ ɪz ɐ pˈiːs ɒv stɹˈɪŋ/

/hˌaʊ lˈɑːŋ ɪz ɐ pˈiːs ʌv stɹˈɪŋ/

used to say that it is impossible to answer a question concerning the size, length, or amount of something

Examples

1. Asked how long this will take, she replies: "How long is a piece of string?"

2. You know, how long is a piece of string?

3. and, well how long is a piece of string?

4. Well how long is a piece of string right.