It seems to me that there's really no good time for the US to conquer Canada. If it seizes it in 1775, then they're driven out a few months later by British forces. The Napoleonic Wars would require the US to at least get a major military complex prior to the war; which would only come about if you had pro-British Federalists in charge.
The Civil War? Eh...
Short of something like TL 191, this seems to be a surprisingly low probability.