Since it is just the beginning, half an hour is enough. It is suggested to focus on the muscles during exercise (think about that part during exercise and feel the changes in that part with your heart), so that you can burn fat better. Some changes will happen to your body after a period of fitness, but when your body adapts to your fitness rhythm and habits, you have to change your arrangement and increase your weight, frequency and duration. But it won't suddenly increase a lot. The standard is to rest for one night after fitness and feel sore the next day.