1. a phrase said when there is no definite answer to a question
you want to know how long it takes? Well, how long is a piece of string?
The expression 'how long is a piece of string?' is often used to answer someone when you don't know how long something will be. A piece of string can be of any length, the amount is undefined, so this question means that you don't know when questioned about the size, amount, or duration of something. This question can sometimes be perceived as a bit sarcastic or cheeky, so be careful to use it with people you know and in informal situations.