Following 24left's lead, I used semi-random dates in EF (Nov 1/18 - Feb 15/19), and note that the lowest's P fare has a 6 month max stay, which somebody earlier suggested might be the case.

24left quoted fare rules before departure, but I think in this case we're more interested in changes after departure. For the fare I found for the dates above, it says for changes following departure:

ANY TIME
CHARGE CAD 250.00 FOR REISSUE/REVALIDATION.
NOTE - TEXT BELOW NOT VALIDATED FOR AUTOPRICING.
AFTER DEPARTURE OF JOURNEY
THE NEW BASE FARE AMOUNT MUST BE RECALCULATED
USING HISTORICAL AND APPLICABLE FARES IN EFFECT
AT THE ORIGINAL TICKET DATE. CHARGE DIFFERENCE
IN FARE PLUS CHANGE FEE. CHANGE FEE IS SUBJECT TO
EACH CHANGE TRANSACTION.
------------------------------------------------
NEW FARE AMOUNT MAY BE LOWER/EQUAL OR HIGHER THAN
ORIGINAL FARE. IF NEW FARE IS LOWER THAN ORIGINAL
FARE - NO REFUND PERMITTED. THE RESIDUAL AMOUNT
IS FORFEIT.
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CHANGES NOT PERMITTED IN CASE OF NO SHOWS -
REFER TO CANCELLATION PENALTY.
IN THE EVENT OF A DOMESTIC/TRANSBORDER AND
INTERNATIONAL AC FARE COMBINATION - INTERNATIONAL
RULE PROVISION FOR CHANGE AND CANCELLATION WILL
APPLY.
--------------------------------------------------
WAIVED FOR THE DEATH OF THE PASSENGER OR PASSENGER
FAMILY MEMBER OR PASSENGERS TRAVELING COMPANION
DOCUMENTAION REQUIRED.