ANDY'S ANSWER: If you can remember when you first noticed your breasts starting to bud, you can expect your period to occur between 2 and 2 1/2 years after that.
There are other signs as you get closer to your period. One will be a growth spurt. Also your breasts will have started to actually grow. You will start getting underarm hair, and you might notice a thick whitish-coloured mucas discharge in your underwear. All of these signs indicate that your period is probably only months away.
You should be prepared by always carrying a pad in your purse or backpack, because you will never know where you will be when it arrives. Or, you could simply start wearing a pantyliner once your mucas discharge appears, and that should prevent you from being caught with blood in your underwear unexpectedly.