I'll be doing the same itinerary late December. I have kept a 3-hour window. It's a 10 pm departure. You'll need to be at the gate at least 45 minutes earlier. You're travelling on a separate ticket, so can't book baggage through. So, slot at least 45 minutes to go collect your luggage and do a bag drop off at QR (assuming you have a boarding pass). Since international counters close an hour in advance, that means you have to do that by 9 pm.

I suggest getting to YUL latest by 8 pm. Work your YYZ-YUL schedule backwards.