"How long is a piece of string?". This is a phrase that's normally used as a response to questions such as "How long does it take to be successful?" or "Why should I climb Mount Kinabalu?" - where the answers are not known or difficult to find.
For example, if someone asked you "What is my purpose in life?", you would respond by saying "How long is a piece of string?" because you don't know the answer.
So, how long is a piece of string? Of course, the logical answer is - a piece of string is twice as long as half of its length - if you know long it is in the first place.
Confusing? Searching for your purpose in life can be like trying to answer how long is a piece of string. It's difficult and we end up following the crowd, even if the crowd is wrong.