When it comes to choosing the best length of a workout, you first need to decide what your particular fitness goals may be. Resistance and weight training is easier when choosing a proper workout length, because the time spent is usually 
determined by the number of muscles groups being worked. The frequency of training your muscles, or the sessions per week, are based upon your skill level. If you are just beginning with minimal skill, you will only train the major muscle groups 2-3 times per week. Therefore, your workouts may not last as long as they will once you reach an intermediate and advanced level of skill. During these phases you will be working out 3-4 or 4-7 times per week. At that point, your workouts could last an hour, or they could last 3 hours.
The best length of a workout for resistance training is going to be user-specific. It is possible to overtrain, so 3 hour sessions can become excessive. Certain supplements provide the muscles with increased amounts of “energy” for training, and these are available at a nutrition center or health store. Anything beyond what can be purchased by any Jonathan or Joe at a local store is not recommended in any setting. If you are training for more than 2 hours, you need to be cautious of what your body is going through. Ample rest between workouts and adequate time for recovery will always be a must for any workout program.
Rest intervals between sets will also be a determining factor in your workout length. For instance, for your general fitness goals, you may rest anywhere from 30-90 seconds between sets. When training for muscular strength, 2-5 minutes is a good rest interval. The more exercises and muscle groups being trained, the more rest time will be required during sets, thus the longer the workout. When looking at all the factors, the best workout length is the one that allows you to efficiently train that day’s muscle groups with a proper number of sets and rest in between sets.
For cardiorespiratory training, the workout length becomes a little more important. This is highly contingent on your fitness level. A 5 minute warmup and cool-down are always recommended. If you want to engage in more steady-state aerobic activity, you can perform a minimum of 150 minutes per week of this moderate-intensity training. That’s roughly 21 minutes per day, and this recommendation is based upon the average exerciser trying to achieve benefits to their health.
Steady-state exercise, or working out at a moderate intensity such as walking or running at the same speed for up to 20 minutes, is a good workout, and calories can be burned. However, recent research is abundant and clear that more vigorous-
intensity exercise burns more calories and produces better results. The reason being that your heart-rate is kept a higher level while you train in an interval-type manner. These interval workouts will be shorter than steady-state exercise, because you are working harder in a shorter time frame. As with any cardio progression, you should begin with comfortable, steady exercise until you can perform 20-30 minutes of continuous exercise at a “somewhat hard” rating. At this point, your heart, lungs, and muscles are ready for more vigorous exercise. Always test your heart-rate during exercise and focus on your target heart-rate, keeping the exercise within that zone.
The best workout length for cardiorespiratory training is contingent on your goals. If you want to increase your overall health, train for 30 minutes to an hour of continuous, steady-state exercise. For increased benefits and results, train for a minimum of 75 minutes per week, with an allowable range anywhere from 20-30 minutes per day of higher-intensity interval work.
