It depends in our house! Tegan started sleeping through at 12 weeks, now it's dependant on what's happened in the day.
I thought it was related to how many naps she'd had, but it's not. Yesterday we let her walk into town and in a few shops and she had a play date, no nap and she was a bit grouchy but went to bed at 8pm and woke up at 8am.
Two days before she went to bed at 7 as she was shattered and she woke up at 4am until 6am and then went back to sleep until 9am! There was no reason behind it, we gave her juice, and she asked for a banana so she had one and that made no difference, gave her calpol in case it was her teeth but that didn't settle her, took about an hour of stories to calm her down and I slept on her floor (hard at 23 weeks pregnant!)
I work 12 hour days and we think it might be a bit of separation anxiety, I'm off work for 2 weeks now so we'll see how she goes.
It is frustrating when we don't know why she's waking up and we're trying to get to the bottom of it!
We've tried pick up and put down, returning her to bed, controlled crying etc.... and nothing seems to work. We're now trying to tire her out in the day more in case that's why she's not sleeping.
Gonna be an interesting 2 weeks!