We often like to put constraints on things for the sake of constraints.
For example, people like to workout for a given period of time instead of working out to accomplish a certain goal. Why is this differentiation important?
Well, if the goal is not just to simply workout for a given time period, but rather to achieve or work on something specific, having an arbitrary time frame you are supposed to workout in makes little sense.
There is no scientific reason why workouts, even though the workouts are different, should last the same amount of time.
For example, when doing heavy strength training, longer rest periods are often indicated, so workouts will last longer. For hypertrophy based training, shorter rest periods are sometimes indicated to help stimulate more muscle growth. So it really depends on the goal.
If we think about each workout having a distinct goal, once we accomplish the goal we should be done. This means your workouts might actually go shorter some days and longer others, but with the hopes of making them more effective in the end.
Train smarter and more efficiently. Sometimes this means training at a higher intensity, other days for a longer duration, and sometimes maybe even a combination of both. Think about what your specific goal or goals for a session are, and then once they’ve been accomplished, you’re done!