There are some good threads around about nursing preemies. It really is a differen experience. I knew I've posted about our experiences in the past so I'm pasting parts of old responses I wrote because I figure it may be older but its likely to be more accurate than my memory is now
Quoting myself ...
"Here's a bit of my story. We started with nursing once a day and otherwise EBM in a bottle. After 1-1.5 weeks we upped it to two nursing sessions a day, then in another week to nursing every other feeding and finally the week before he was due we "graduated" to all nursing all the time. Yay! Before that time, I pumped everytime he took a bottle and 3 hours almost around the clock until we went 100% to the breast. Boy, I was glad to finish that!
All of this nursing was done with a nipple shield to make latching easier. Also, we offered a bottle at the end of every nursing session, including the first week we went 100% to the breast, to ensure that nursing sessions weren't ending just because he was too tired to suck anymore rather than because he was full. It sounds like you're doing this too and that your son keeps going until he is full and I think that is a good sign.
We did a few weeks of 100% nursing on the breast with the nipple shield and then worked on ditching the shield. He did fine latching off of the shield but then we ran into some other (non related medical) complications that made our nursing relationship rough again. But I can say that today, at 5.5 mo (4mo adjusted) I have a good supply of milk and he's nursing great."
My Dh was in a rush for us to ditch bottles and get 100% on the breast but honestly, Ds needed time to grow and become strong enough to fill himself by nursing. I felt this slow, comfortable pace for him was an important part of helping him adjust to the world without triggering fear around hunger. In hingsight, my Dh respsects that this is what our Ds needed, even though at the time he felt anxious about getting started with nursing."
And another post I wrote:
"Preemies get really worn out sucking. If babe is nursing too long she's burning as many calories trying to bf as she can take in. I'd suggest limiting nursing sessions to 30min max so babe doesn't wear herself out. Mom should pump to stimulate and baby should be offered ebm by syringe or bottle after every single feeding.
Obviously feed on demand. Feed until she stops, but IMO, don't try to force her to continue. Rest is really important, just as important as food. Actually, 0.5oz sounds like a good session to me (for a 35 weeker at 37weeks). That tiny tummy might really be filling up on 0.5oz. Think in terms of daily intake, not per session. Assuming the babe is still really sleepy, I would not wake the babe more often than every 3 hours to nurse. This will give her time to re-coup her strength before the next session. "
Just to add a few more details: Our limits were, IIRC, nurse no longer than 20min OR until baby stops the suck-swallow-breathe sequence (meaning the pauses get real long signaling baby stops eating). Always offer a bottle immed. after 20min or when baby seems to have dropped off to sleep. Feed on demand but never wake babe sooner than 3 hours after the start of the last feed (i.e. baby should get 2-2.5 hours of sleep min between nursing sessions to recharge). Ds never would suck on our pinky finger, he thought we were nuts to even offer.
Weaning from the bottle: We recorded the volume of intake by bottle and tracked the volume by the day and looked for increases there rather than session by sesssion. As he neared his due date, Ds was still taking in a lot from the bottle, then spitting most of it back up (i.e. it became clear he was sucking on the bottle because he liked to suck and was becoming overfull because he couldn't stop the flow, kwim?) At that point we slowed down to offering the bottle every other feeding session, then just twice a day, then just afternoon/evening when Ds would be more wornout, then only if he seemed hungry and my breasts were 'empty' (i.e. he wasn't strong enough to extract any more milk). We waited 3-4 days between the different phases. I'm pretty sure we weren't completely weaned off the ebm bottles until right around EDD or even a week or two later.