Duration refers to the length time spent in an activity. Effective duration is the productive time of an activity.
In exercise, duration is the length of time spent in a single rep, or a single workout session.
However within one repetition parts of the motion do not contribute anything to your success.
In a workout session, many activities take time and add to the duration of the session without any significant impact on you results.
Recognizing the top 20% of activities that contributes to 80% of your result is the easiest way to save time.