Most people’s workouts last around 60-90 minutes. While that is common practice, most of the time spent during such workouts is actually resting between sets, which is to long. Longer resting than necessary will undoubtedly lower the intensity of your workouts and therefore decrease the chance for muscle growth. If every set that you perform last in average 30 seconds, and if you rest only 30 seconds between the sets, you will be able to complete 20 sets and achieve great intensity in only 20 minutes! Not many people perform 20 sets per muscle group any way, but if you are well trained and conditioned, this is the type of workout that you want to do.
If you want to train 2 muscle groups per day, my advice is to do one 20 minute workout in the morning and another one in the evening. You can also do only one body part per day, depending of your level and ambition, but the bottom line is that you don’t need more than 20 minutes to complete a single body part in a workout. Give it a go and watch your muscle grow!