I expressed exclusively when DS was 3 weeks old and my appendix burst and was rushed to hospital. The three days after the operation I expressed every 2 hours round the clock with a hospital pump. I was determined to BF him so I pumped to keep my supply up, and then I just had to tip it all down the drain because of all the medication I was on. It was horrendous.
I expressed exclusively when I went back to work full time at 6 months, having exclusively BF from the breast before that. I got about 300ml each pumping session which I now know is loads but seemed like almost nothing at the time given the rate DS was going through it. I was pumping at the weekend as well as feeding him from the breast directly to build up a freezer stash for the week ahead.
It was awful, so awful I only lasted 4 weeks. I was constantly stressed and worried that I wasn't producing enough for him, my supply would tail off, I was wasting time at work pumping etc. I had a great electric double pump which looked like a rucksack and I would recommend to anybody if they really need to pump - but on the basis of my own experience I would never advise anyone to even consider exclusively expressing, unless there's a genuine medical need.
Finally I basically just trusted to my body to adjust and we went down to morning and evening (and night-time) BFing from the breast, and two bottles of formula during the day. Thankfully my supply held up just fine. DS is now nearly 11 months and we're able to have whole days together with no bottles, just BFing on demand, and also days where I am at work and he has four bottles over the course of the day.