Research shows that you will probably get the best output with a suction intensity that feels strong to you, but is not painful. Start at low intensity and gradually increase suction to what is comfortable for you. You may find that you can increase suction intensity over your pumping session as your nipple tissues become more elastic.
Pumping should not hurt! If you are experiencing pain while pumping, you may need to experiment with lower suction over a longer time, try a different flange size, or try a small amount of pumping lubricant.