Now that we have the opinions out of the way, let's get to the facts

It depends on the fare rules of a given ticket, which can vary greatly. You need to check the fare rules, as was mentioned. I checked a randon fare rule for a random date NYC to LON.


16. PENALTIES
UNLESS OTHERWISE SPECIFIED NOTE - RULE 16N3 IN IPRG
APPLIES
BETWEEN THE UNITED STATES AND UNITED KINGDOM
ORIGINATING THE UNITED STATES -
CANCELLATIONS
ANY TIME
TICKET IS NON-REFUNDABLE IN CASE OF CANCEL/NO-SHOW.
WAIVED FOR ILLNESS OR DEATH OF PASSENGER OR FAMILY
MEMBER.
NOTE -
1. RESERVATION CANCELLED PRIOR TO THE ORIGINALLY
SCHEDULED FLIGHT RETAINS TICKET VALUE/LESS
APPLICABLE CANCELLATION FEE IN CHANGE PARAGRAPH
UP TO ONE YEAR FROM THE ORIGINAL DATE OF TICKET
ISSUE. IF THE TICKETED FLT RESERVATION IS NOT
CANCELLED PRIOR TO TICKETED FLT DEPARTURE TIME/
REBOOKING IS NOT PERMITTED AND TKT HAS NO VALUE

So in this case, cancellation up to the scheduled flight departure time retains the value less the change fee. After that, and with "no-shows" you lose the full value.