I read something somewhere that said "90% of babies sleep 6 hours or more by the time they are 3 months old." And for what it's worth, in magazines/books and according to doctors, 6 hours is considered sleeping through the night. It doesn't feel like it if you put them to bed early, though!
Our experience with Emma thus far is that we put her to bed between 9:30-10:30pm. She then wakes around 3-4am to feed. So officially she's sleeping through the night... We're still hoping for longer stretches as she nears the 2 month milestone, but we shall see. I'm pretty happy with what she's doing now!