I had my son @ 26 weeks & the lactation specialist where I'm at said drink water, Sleep, don't go more than 5 hours pumping and pump 8-10 times a day. As far as how much, they have this grid and by 3-4 weeks you should be pumping 800-1000 mL in 24 hours, so if you haven't started I'd suggest starting to keep track each pumping how much you produce. They also said that delivering so early that your body will need extra time to actually produce what it should so don't give up. I was about to give up at 400ml per day but now im almost to 700mL a day . I get the most when I sleep good, hydrated, and drink mothers milk tea(although it's not the best tasting). Hope this help, best of luck to you and your little one.
I think that there's a point, about two weeks after a preemie is born, when the physical stuff can just really start to slam a mama. Your body has some reserves, and that's about the time when they start running out.
Drink water. Eat. Rest. Keep pumping (although I am in favor of a long stretch of sleep overnight). Make sure you always have water with you, and put some snacks in your bag as well.
Tell anyone who asks if they can help to take on your housework, or bring you casseroles.