A minimum aerobic exercise should be 12 minutes.Why?
That’s the time organism needs to activates enzymes responsible for transforming fats into energy. So, after 10 minutes of aerobic training fats increase their role in energy production, while carbohydrates and glycogen reduce.
The largest effect of having fats transform into energy happens at 30 minutes. The reason for that is that our nerve system uses exclusively sugar, or actually glycogen. After 12 minutes the organism initiates a self-defending reaction to protect itself from eventually emptying all reserves of glycogen, and starts using more complex sources of energy, primarily fats. For this reason you want to exercise a minimum of 1 hour, or over 30 minutes at least. Aerobic training longer than 1 hour also activates fats for energy, but not the same as within a first hour. A training over an hour increases a risk of injury.
The time you spend resting, sleeping enough, your daily food consumption and current feelings are important factors for determining longevity of your training. The results will for sure be better if you exercise longer, but the training must be tailored for you and your condition.
It is very important you do the warm-ups. Warming up and relaxing after you train must be an integral part of every training. Your pulse must be under 55% of maximal theoretical frequency, and you need to slowly raise heart frequency towards your training zone through 5 to 10 minutes. Every training, and exercise start slowly and systematically, and equally finish it the same way.
Only that way you will allow your organism through regulating blood stream and metabolism, to prepare itself to jump from one level of intensity into another, what increases the effect of training. Entering too fast into training zone can produce sudden jump of heart rhythm, sudden loss of energy and too soon tiredness.
Slow ending and relaxing will allow organism to eliminate lactic acid and reduce a chance of muscles inflammation.