Can't speak of how things look on a Sunday vs. any other day, but prior reports on FT that the UAL check-in agents at YUL also support gate operations and are known to abandon the check in area up to 45-60 minutes before departure time.

I would assume at least 30 minutes for customs, but again this is dependent on time of day. Seem to be many US bound flights leaving early morning and again mid-late afternoon; may require more time.

My recent YYZ experiences by comparison involve afternoon customs and security delays commonly in excess of 1 hour. My general experiences at YUL have been more positive.