Early morning classes: 45 minutes - get 'em in, work 'em, get 'em out. Fast!
Lunch hour classes: 30-40 minutes - as above, but faster.
Evening classes, day, weekend - anywhere from 45 minutes up to 90 minutes.
A lot depends on the purpose, wants and desires of the participants. I taught a 60 minute class (my preference, my regulars preference) in the evening that was preceded by a 40 minute class and then followed by a 45 minute class. All three were popular with participants. The combinations were required to fit the flow of other classes in the facility and to accommodate the downtown work crowd as they left their cubicles to head home.
I like 60 minute classes. Of course, I am somewhat long time in saddle rider (my short Saturday rides outside in the rain and cold only last 3 hours, summer, longer).
A 60 minute class means I can do a nice long warm-up to prepare them for the work-out. Or I can provide a variety of mini-workouts. The masters swim coaches I am swimming with do a 40 minute warm-up/build to the main set of 15 minutes work, for a 60 minute session - so that concept is also an option.
60 minutes is a long time for new participants. They may need to take it easy(easier) for part of the class, or even leave early. Everyone gets permission to do what they need to do for themselves.
So - class length really depends on the crowd and the purpose. For a class geared towards new riders, 30 minutes of ride time, 10 minutes of set-up time might be great. For experienced riders, 90 minutes may be what they are looking for - provided they have the time in their schedule.