My commute is too short to qualify as exercise. In summer it can be as short as 12 minutes if there's a tailwind. In winter it's only 20 minutes, but done slowly as it's on lumpy snow and ice. (I don't actually ride much in winter anyway.)

The exception is when I ride to the pool before work (something I don't do in winter as the roads leading to the pool aren't very safe then). Then I can get in 15k (25-35 minutes) plus 30-50 minutes swimming plus another 20 minutes to work. Or I sometimes add in laps of a 5k circuit near my house on the way home just to increase the distance.