It can take a year for a girl to fall into a regular pattern. Your DD may go months before another or she may have another in 3 weeks. Teach her to be prepared. If she's lucky, she'll normalize pretty quick.
This is off topic a little, but it might be a good idea to teach her how to make a makeshift pad using tp just in case she is ever in a situation where there are no pads. My DD ran out of pads on her way home from camp and was totally unprepared for handling that. Your preparation comment reminded me of this.