It is a philosophical sort of question and it's impossible to give a good answer to it. In every case the amount of training sessions per week must be adjusted to individual predisposition. Quite different are needs of a gym novice and of well-experienced athlete. As a rule, you should also specify what effects you're talking about. A very bad habit is to use colorful pictures from the press or from the Internet to state: "I want to look like this girl/guy".
You should definitely read:How to quickly build muscles. The Ten Commandments for mass: part 1
Too often pictures shown on the Internet are improved in graphics programmes, and to make matters worse, people presented in these photos have long-term training experience.
You can not change the quality of your body from overweight or even obesity to a slim figure in a few months (as it is promised by tabloid articles with supplements advertisements). Years of neglect and build body fat will also take years to eliminate. Each kilogramme of fat is a source of nine thousand calories. A man weighing 91 kg, during a one-hour run at the pace of 16 kph will burn only 1125 kcal. Bodybuilding workout (70-80% of maximum resistance) for a man weighing 100 kg will burn around 630-700 calories per hour. Let's calculate: body fat reserves of an average man weighing 91 kg will be enough for 105 hours of continuous run at the pace of 16 kph. This means running 1680 km - this is not a typing error; say: one thousand six hundred eighty kilometers! When caluclated per year it means running 140 km per month - or 35 km per week. A bodybuilding training will require at least twice as much time. A lighter person and employing lighter activity will burn body fat more slowly. I am deliberately simplifying the use of body resources in the from of carbohydrates (muscle glycogen) which further complicates the issue of drawing energy from fat by your organism.
80% of the work on the perfect shape (no matter what your target in training is: strength, weight, fitness, fat reduction) is done in the kitchen, not at the gym! Without a change in eating habits you will need to spend many more hours to obtain satisfactory results. Certainly, if you start to train and eat in accordance with the intended objective (eg. fat burning), you will see results soon than when you sit and do nothing. If you have led sedentary lifestyle, you can not train every day, not to mention twice a day. Such a training will quickly lead at best to overtraining and fatigue, and in the worst: to joints injury. Start with the smallest possible amount of training a week: for example optimal are 3-4 sessions a week lasting no longer than 70-80 minutes (including a warm-up and cooling down after your workout). Strongly recommended is an adaptation training for instance in the form of marches, cycling or similar activity performed for 4-6 weeks, 3-4 times a week for 20-45 minutes (duration and intensity must be selected according to your physical abilities). Such preparation definitely reduces the shock you will experience after introducing a strength training.
If you want to get rid of body fat, I'd plan your week this way:
- 2 x weight training based on the FBW model - 60-80 minutes followed by 15 minutes of for example aerobic training (eg. bicycle, treadmill).
- 2 x jogging or running for 15-20 minutes (depends on the duration of training adaptation and consequently your fitness). Gradually increase the intensity (eg. increase your pace and training duration).
Strength training boosts your metabolism, helps to increase muscle sensitivity to carbohydrates (you will tolerate supplied carbohydrates better, eg. bread, rice, cereals and groat - this means less deposition of body fat), improves muscle blood flow (better transport of nutrients) and allows your body to better utilise tissue fat.
Running or similar training will help you burn body fat. Gradually and very slowly increase the time (for example add 3 minutes per running session). Similar recommendations refer to the increase of workloads in strength training. Adjust the weight so as to perform the planned number of repetitions (the recommended range is 8-12 reps.)
Sources: „Fizjologia człowieka z elementami fizjologii stosowanej i klinicznej”. Władysław Z. Traczyk, Andrzej Trzebski; Wydawnictwo Lekarskie Pzwl 2004 2. „Fizjologiczne podstawy wysiłku fizycznego”, J. Górski Wydawnictwo Lekarskie Pzwl