When do you start pumping ?

This is my second child, first one went through breast milk and 3 formulas before we found one she could keep down. I know breast milk has a lot of benefits so I wanted to try again. I’m not a fan of my nipples being touched so I was able to order a breast pump through insurance to exclusively pump. My question is, when do you start? I’m due in 3 weeks and have no idea where to start. Do I pump now to build up a supply? Do I start as soon as baby is born ?? I’m so confused as to how this works. Also how long does baby drink breast milk? A year? 6 months? I plan on going back to work after a few months and I’m not sure if I’ll be able to pump since I work with food and small children. I don’t have a lactation coach or anything. I was planning on formula feeding but want to give this a try. Thanks