Hope, The first year can be VERY irregular. This is quite normal. She might not even be ovulating every cycle. How old is she and is her health and body weight ok? Is the rest of her sexual development happening as it should?
If you are very worried about it, get her to a doctor. Also encourage her to keep an accurate calender of her periods. It is very useful to have an accurate record when you talk to a doctor about it.
Take care.