New to exclusively pumping no breastfeeding

So I’m having trouble with the minuted each session I know I just pump every 2-3 hours to make sure milk supply doesn’t go down but I do 15-20minutes and my bottle has 120ml but I usually pass that line for each breath so I produce atleast 280ml each 15-20 min and I thought if I pumped only 2x a day and jus produce atleast 25-30 ml I’m good but then my breast start getting hard and I start leaking so I need help with how many minutes I should I pump and if I produce very well because I know I must do Atleast 6-8 sessions a day my baby takes 2-3 ml every 2-3hours bottle fed breast milk and I know I might need to freeze because of my oversupply I was jus asking if it matters how many minutes I pump each seasion because I was thinking I can produce 2-3oz each breast each session because it usually what he eats .