These pirouettes should take about 16 seconds to complete a full 360 degree rotation. Try to keep the helicopter stationary while performing the pirouettes. Be sure to practice both clockwise and counterclockwise pirouettes to avoid developing "handedness".
The slow pirouettes are particulary good for diagnosing your own orientation weaknesses because you will lose control of your slow pirouettes at your weakest orientations. You should practice these weak orientations to improve them.