You’ll be unsure and ask friends ask professionals, and you’ll get a full spectrum of answers. I’ve heard every timing under the sun and not much to back it up.
So here’s another input to add to the pile, this is what ended up happening for me: First time I pumped was at the hospital with their super duper pump; day 2 and my supply has been good since. Related, who knows? I tried pumping at home and I hated it. Instead at home I got shells that catch leaks from the other boob when feeding her and started storing in ice tray in the freezer. (Which we never used because she never took to bottles.)