Young girls start their periods at various ages. Approximately how long does it take to be regular (i.e. roughly monthly)? If regularity does not occur, then at what point would seeking a doctor's advice be appropriate.

I ask this as a single father of a teen girl who is not currently regular, but had her first about 2 years ago. She's had 2 or 3 in the last year. The last was around 4 months ago.