Conversation so far:
When did John Lennon meet Cynthia Lennon?
John Lennon met Cynthia Powell in 1957.
Did John Lennon meet Cynthia Powell while attending school?
John Lennon met Cynthia Powell in 1957, when they were fellow students at the Liverpool College of Art.
How old were John Lennon and Cynthia Powell when they met?
Cynthia Powell met John Lennon when she was 18 and he was 17 years old.

Question: How long before they married?
Self-contained rewrite: How long before John Lennon married Cynthia Powell?