Another Mom here of premie twins who worked hard to establish BFing. My story is very similar to many I've read above. I think it took about 2 months before they had transitioned to exclusively nursing.
They were born 34w2d and spent ~1w in the NICU. They came home on bottles of EBM supplemented with formula, and were so sleepy it was hard to get them to eat anything - let alone to nurse. After 1w or 2w I started with a nursing session a day, but I really didn't know what I was doing and neither did they. Finally we hired an LC to come to the house and she made all the difference for us.
I have to admit that I am very pleased that we were able to succeed on the BFing front. It sure was tough in the beginning, and I can certainly see how many women would wind up stopping for a myriad of reasons. I was fortunate to have the support of my DH, my Mom, and the resources on spring on an LC (I said it was money we'd otherwise spend on formula). I was patient and persistent but still there were crying moments during that transition period (and I mean ME, not just the babies!).
During the transition period I first replaced daytime bottles with nursing for both babies. I made the leap to 24 hours with first one baby, then the other. Considering they each nursed for 45 min at a time I was hesitant to nurse both overnight until *I* was more comfortable with the idea. Once I was able to nurse them tandem then nighttime got easier.