From this article Short workouts: Will exercising for 15 minutes once a week get you fit?
“The key to the short workout’s success revolves around a concept known as high-intensity interval training (HIIT). HIIT is a heightened form of interval training that involves alternating between periods of short, intense physical activity and fixed periods of low activity or rest. Intervals can include anything from fast squats and pushups to weight lifting and powerful cardio.”
The goal in exercise is expose the body to more than it is used to handling. If given enough time for recovery the body as a form of self-protection will come back stronger, more enduring, and more able to withstand the stresses previously placed on it. That can take one hour or two, or it can take less than ½ an hour. Regardless of the time period at the end the session you want to come out spent.