I know that during the era of War Plan Red on the American side, Britain effectively ceded Canada as a lost cause in its own postulations of a war with the United States. American forces were assumed to overwhelm Canada, and Britain would therefore focus on beating the United States on the ocean.
That is not actually ceding Canada. The assumption was that the USA would only go to war in Canada on the assumption they would go to total war. This, you may note, is also the assumption of the great majority of War Plan Red contributors as anything less leaves the door open for an embarrassing defeat at the hands of British Army that was still accruing combat experience in various Imperial campaigns.
Given this assumption it was decided the place defend Canada, save Nova Scotia which is a special case, was beyond its shore line. The US economy relies on trade, interrupt that trade as the British well could till the 1940s and holding onto Canada becomes very expensive for the Americans. At which point they can be induced to yield it up, possibly for some minor concessions but maybe in totality plus reparations depending on the exact political circumstances. Nova Scotia having only a very limited land frontier to defend and of course the major naval base of Halifax would have been defended on land because the forces required could still have been supplied and reinforced by the Royal Navy and Merchant Navy.
It should be noted that said interruption would not likely have been a total blockade like 1812 but rather more particular commerce warfare. However the simple impact of prolonged denial of British hulls for shipping purposes would have been costly and the US merchant marine post World War 1 was itself a much larger target and more importantly a much more significant carrier of US goods by this time period. Thus the calculation that the costs of a prolonged sea war were greater to the US than the gains from Canada.
So the simple answer to the question is never. For by the 1950s a single A-bomb delivered to Wall Street would also have cost the US economy more than it could extract from Canada above and beyond normal peace time trade. Not that by that stage it was likely to come up.