It might depend on motivation, as I don't really think of any ride much under 45 minutes to be much of a workout even if I do it twice a day. That's about how long it takes me to ride my 9 hilly miles to where I catch my bus, but if it was shorter I would need to do longer rides home more often than I do now. When I run I rarely run for less than 45 minutes either, though that's only once per day.