Breastfeeding my little one in the NICU

Hello everyone!! My son was born on May 3rd, he is having little problems with breathing (randomly his O2 level will drop and he turns blue around his mouth). So since I really wanted to exclusively breastfeed him I am here with him every day!! He is eating great, every two hours about 10 minutes on each breast. And my milk has come in (with the help of a little pumping), but I have a pumping related question. So like I said I was pumping after each feeding. I stopped pumping because my breast felt to engorged, and I don't know when it is a good time to start pumping to start making the stockpile. I know pumping in the morning and at night you get the most amount of milk but when should I start this process I have to go back to work and my mother is going to be watching my son so the sooner the better.