please do not only pump once to avoid engorgement unless you wish to wean off baby soon.
I am a low milk mother but still, I express twice (11.45pm & 5.45pm) for 30 min when at work. Also, I do not time the pumping duration of each breast, I will express alternately (ie, if left breast express till no milk, I switch to right and then left again, right again...) Then, before I finish the session, I will use my hand to squeeze the little milk left in the breasts out to totally empty both breasts...cos sometime breast pump takes ages to express the remaining milk inside towards the end. You will be surprize that you can squeeze about 10ml ~ 20ml out.
Hope my advice is helpful to you.