There are various guidelines relative to various qigong systems. The one thing you can count on, is how you feel. You can tell if you are depleted of energy or not. There are side effects to low energy, like lower back pain, or kidney soreness, ringing in the ears...
Balance is struck by the individual, relative to a systems guidelines. I find SFQ is very user friendly. Many of the meditations/exercises gather new energy "in", and the positions are not too physically demanding, and allow for variation in how they are performed. (lying or sitting, versus standing, or standing in a lower stance)