When will my extent start? (ended dec 30)?

the last time i started my spell was on december 24. it last until december 29.
i dont know if when u say 28 days, does it plan from when it starts or when it ends? all girls are different u might skip some months its usual.
Usually it is about 28-30 days until your subsequent period BEGINS. SO... If yours done the 29th of December you'll probably get yours between January 26-28th.