Objectively, how long would it take for the USA to take over Canada?

 Allan:

A pentagon study done after the Quebec Crisis and Trudeau Sr. threatened to close off the St. Lawrence Seaway if the US Army crossed the border showed that it would take 24 - 48 hours to assume control of the country. 

The problem would show itself on day three when they attempt to occupy Canada. 

The estimate was that it would tie up 2 million troops within Canada where policing a similar people would demoralize their troops and another 1 million troops to protect strategic interests in the US from pissed off citizens or descendants of Canucks. 

Conclusion: Easy to invade but impossible to take over Canada.

K.C. Armstrong