I have a simple question on a stopover rules.

Planning a LON NYC vacation in November with DL

03 NOV LHR CDG JFK AF DL
09 NOV JFK DUB LHR DL EI

Coming home i was planning a stopover in DUB to visit friends... For how long can I stopoff before the fare goes from a very reasonable (£235) L to Y (£800+).

Pity the L class is not upgradable!