AMOUNT OF PHYSICAL ACTIVITY
The Nutrition Source by Harvard T.H. Chan have analyzed that if you're exercising to lose weight, then a 30 minute workout everyday and eating well should be effective.
Children or Adolescents: 1 hour or more a day
Healthy Adults: Minimum of 2 - 1/2 hours of moderate to intense aerobic activity per week. 5 hours of intense aerobic activity would provide more health benefits.
Adults should also aim to do muscle-strengthening activities at least two days a week
Older Adults: They should be physically active as their conditions allow.
For those who are new to exercise or haven't done it for a very long time, you should meet with a trainer 2-3 times weekly.
This ensures proper form and muscle memory.
For those who hit the gym pretty often but stopped seeing results, you may want to consider working with a trainer for one-two months. After one - two sessions per week in the first month, you will start to see a noticeable improvement.
HOW MUCH IS EFFECTIVE?
Depending on your fitness goals, you'll need to commit to a minimum of 2-3 days of exercise each week. The ACSM recommends 150 minutes of moderate intensity cardio, 75 minutes of high intensity, or a combination of the two per week.
Resistance training and Flexibility should be practiced at least twice per week.
At most, you should be hitting the gym 6 times per week total (weight training, cardio, etc..) and taking 1 full day rest.
Note that this should be specific to you and your health goals. Some people might only need to exercise at most at 3-5 times per week.
WEIGHT TRAINING FREQUENCY
The majority should weight train 3-4 times a week, and not more than 2 days in a row to allow optimal recovery and the best results.
MUSCLE GROUP FREQUENCY
Most people expect to gain much more muscle faster than they actually can. Men become disappointed when they can't gain 12 pounds of muscle per week, and women are afraid that they'll gain too much too fast and become bulky like men. On a related note, women should be working out EXACTLY like men.
One of the most common frequency is training each muscle group or body part ONCE PER WEEK. However, this is the LEAST EFFECTIVE as backed by science and qualified experts.
The MOST effective training frequency for beginners with any goal or those interested in strength building is 3 TIMES PER WEEK.
For those with other experience levels is training each muscle group TWICE PER WEEK.