How long is a piece of string?
Definition 1 of 1
Pro Tip 1/3Uncertainty Context
Use when an exact number or amount cannot easily be given or is unknown. 
How long will the meeting last? Well, how long is a piece of string?
Pro Tip 2/3Non-literal Use
Understand it's a rhetorical question, not expecting a literal answer about string. 
How successful will the project be? That's like asking how long is a piece of string.
Pro Tip 3/3Avoid Overuse
Use sparingly to avoid vagueness in situations where specifics are important. 
When budgeting, instead of asking 'How long is a piece of string?', seek exact figures.