Ok, so I realize this is a problem most people would love to have. Please don't hate me!
DD is just over 3 weeks (23 days) old, we EBF, and she's quite a good sleeper. During the day I have to wake her to feed her every three hours, otherwise I'm pretty sure she'd just keep on napping. She's usually fussy in the evening and cluster feeds before finally going to sleep between 11 and 12. She has STTN for eight hours twice.
Until last night........I fed her from 7 until 8 (slow eater over here!) and she fell asleep pretty much immediately thereafter. She was still sleeping when we went upstairs at 9:30-ish, so I took her with us and put her in her bed. I went to bed hoping to get in an hour of sleep before she woke up to eat again. Cut to 4 a.m. when she was STILL sleeping soundly and I woke up, mildly panicked, covered in milk and painfully engorged.
I decided to wake her to feed her, as she'd been sleeping for 8 hours at that point. She ate, went back to sleep at 5 and slept until 7:30.
WWYD in this case? I thought waking her was the best idea because (a) she must have been hungry, (b) my boobs HURT!, (c) I chatted with my midwife last week and she gave the okay to let DD STTN (for eight hours) if/when it happens, but I figured I shouldn't let her sleep any longer than that at this age.
For the record, she's gaining well and has more than enough wet/dirty diapers. Oh, and she was over two weeks late, so developmentally she's actually considered to be five weeks old (obviously still not old enough to sleep for such long periods of time).