When Natty was very little, it was well defined - from 7pm until 9 or 9:30. She would just cry and cry. All we could do was give her cuddles and wait it out.
Now there are so many variables, LOL! It depends on how much sleep she's had. There's no real set time for us these days, except that there's always a 'time' of the day when she's cross and nothing I do is right.