I was considering the same question a few days ago when booking a reward ticket with stopovers in Europe.
I agree with the above comments about different airports, different seasons, etc., but the rule of thumb I'm using is I want at least a two-hour layover. Nothing magic about that amount of time; just general experience tells me a flight delay is very likely to be less than an hour, with some rare exceptions.
I saw Aeroplan offering options with one-hour layovers. I wouldn't take the chance, personally.