Hi. My DD was much smaller than yours. She was born at 31+1 and was 3 lb 3.
I expressed every three hours. Sometimes slightly less frequently overnight if I was totally exhausted. I think they recommend expressing at least eight times a day to build up and maintain your supply.
I second the suggestion of hospital grade pump.
Does your NNU not have breastfeeding specialists you can talk to?
I think all babies are different and a lot depends of why your LO is in NNU and being tube fed.
With us, we first got DD onto 'normal' bottles, with a teat that we could push into her mouth, and that dripped a bit without her having to suck much. Then we moved onto the tommee tippee closer to nature bottles which taught her to open her mouth and to suck/manipulate the teat to get milk.
Whilst doing this I tried her at the breast once or at most twice a day. She was tiny and attempts at bf were really tiring fir her (because we then had to bottle feed her).
Eventually she just clicked and started latching on to me. This was when she was about 37/38 weeks, and coincided with her being much more awake, so I think she was just ready for it.
Dont know if any of that is helpful.
One tip we used was to bottle feed milk which wasn't warmed. At warmest it was room temperature. This meant that once she could latch on, she had a breast preference because she was getting nice warm milk straight from mummy, rather than the slightly colder stuff from the bottle.
I think that the tommee tippee bottles were as much work for her as the breast.
Congratulations on the new arrival and I hope your DS us home soon. DD is just coming up eight months now and breast feeding is going strong so it is possible, if not easy.
xx