Conversation so far:
When did John Lennon meet Cynthia Lennon?
Lennon met Cynthia Powell (1939-2015) in 1957.
Did they meet while attending school?
in 1957 when they were fellow students at the Liverpool College of Art.
How old were they when they met?
CANNOTANSWER.
How long before they married?
The couple wed on 23 August at the Mount Pleasant Register Office in Liverpool.

Question: How long did their marriage last?
Self-contained rewrite: How long did John and Cynthia Lennon's  marriage last?