In hospital I was told to do this every 3 hours. Usually dd (6 days old) wakes before the 3 hours is up, but at what point can I lengthen the time between feeds if she's still asleep? Especially during the night
I'm sure that when I got DS weighed at about 2 weeks I asked the same thing and the HV said feed at least every 2-3 hours during the day, feed again at 11pm then just leave baby till he wakes. DS was very small 5lb11 but had re-gained his birth weight by 10 days, so HV wasn't worried provided he continued to gain weight appropriately.
So in answer to your question, I would continue waking every 3 hours until you are sure she is gaining weight appropriately.
I have heard that for a healthy full term baby who was not low birthweight, once they have regained their birthweight you can let them sleep longer. I never had this issue - DS woke before 3 hours was up, and he still does 7 months later.