Regardless of the mode, anything over an hour appears to be the point where people start to find their commute to be onerous although cycling for an hour a day is probably going to be more enjoyable than driving or taking transit so one could extend that a bit.
My commutes have run from 30 minutes to 2 hours one way (50km)... riding 100 km a day (62 miles) was actually very enjoyable and something I did in good weather with 80 of those 100 km being ridden in the country and not in the city.
I did this in my mid 40's but came into that with a good deal of experience in long distance cycling... I used to average 60 km a day as I commuted and also worked from my bicycle to make calls on clients.