Hmmmm... this is a tough one! Firstly, congratulations! Secondly, big hugs on enduring the NICU experience right now. Thirdly, KUDOS for pumping around the clock and thinking of others while doing so!! :o)
Like ~Adorkable~, I also had twins. Mine are almost 4 months old. They were born at 33 weeks and were discharged at 37. When we left the NICU, they were breastfeeding but not exclusively like I had hoped. So, I continued to breastfeed and bottle (which obviously meant pumping) until they were solely nursing for all of their feeds. We had to bring a huge cooler to haul all of the frozen milk home that was stored in the hospital. Then once we were home, the stash continued to increase and our deep freeze is overflowing. After they began nursing exclusively, I quit pumping altogether. However, in my opinion, I think the needs and demands of milk for twins is quite a bit different than a singleton.
With that said, I think there are a lot of factors to consider. For me personally, I probably wouldn't consider donation until your LO is discharged and exclusively breastfeeding for awhile. The NICU is such a crazy place, and while I'm sure everything will be smooth sailing for you, there are so many variables that you can never plan for (or even outside events like what ~Adorkable~ mentioned). I wouldn't want to be in a situation where something happened that was extremely stressful and directly affected your milk supply and then you were left wishing that you had kept your precious resources! ;o)
Also, I don't know what your plans are after you bring your baby home (SAHM, work, etc), but those circumstances may come into play as well. If you're a SAHM and your baby is able to always nurse on-demand, you probably won't end up using much of the pumped milk. But if you go back to work, you may find that you may not be able to pump as much as you'd like or your baby eats more, etc, etc. It's tough to know.
Like others mentioned, I would talk to the milk bank to see what their requirements are. Preemie milk is different than term milk, so I don't know if that has different requirements or not.
Best wishes to you and your daughter! Hopefully your time in the NICU is uneventful and just spent feeding and growing! :o) Big hugs mama! Stay strong. It can get monotonous and lonely - but eventually you'll be on the outside! Sending you smiles.