Meaning of the phrase "How long is a piece of string?"

Hello,

Can you please tell me the meaning of the phrase "How long is a piece of string?" when put into context of the question "How much do you miss me?"

Thank you so much.

Patricia

Intrinsically a piece of string has length but that length is unknown hence the
the phrase 'how long is a piece of string' means that the quantitative answer is not known and there is an implicate understanding that the answer will be difficult to find given the information available.