I'm looking at a flight that goes from the US to Europe via Montreal. Is a 52 minute layover in Montreal enough time to get to the next flight? Arrival time in Montreal would be about 8PM.
After having gone through border control – primary inspection, you will be asked to follow one of two procedures:
If your airline offers the option of automatically transferring your luggage, you will be directed to the international departures area.
If your airline does not offer the option of automatically transferring your luggage, you will have to pick it up yourself from the carousel, then follow the normal procedure for international arrivals and then for international departures.
For USA->International connections, you need to find out if your airline offers the option of automatically transferring your luggage.
If that is the case, you don't have to "clear customs" in a traditional sense at all (in Canada) and you have to simply walk over to the international departure gate. You shouldn't have a problem, unless your flight is delayed. You still have to go through "border control" (speak to a Canadian border agency worker, provide evidence of transit visa or lack of need for one, attest you aren't carrying certain goods, etc).
If it isn't the case, then you will have to get your luggage, clear Canadian customs, and only then proceed to the departure gate. This will be much trickier to pull off.
Possibly, if you didn't have to re-check your luggage. Canadian immigration is pretty speedy. But it's a tight connection, and a small amount of bad luck could easily lead to a missed flight, and then you'd be stuck in Montreal overnight (a pleasant place to be stuck, but still). I would never ever try it unless it was a single-ticket, guaranteed connection. (And I also wouldn't do it unless I could easily afford the extra costs for overnight accommodation and whatever messes it would make for my destination plans.)
Air Canada note that 40 minutes is the minimum time, although they note this only applies for Air Canada/Jazz flights. I'd personally want to allow more time though