This means for the next [average] 2 years after you first began your period, you could go a week, month, 3 months, 5 months, or even a year without your period. One time your period may last for
3 days, another for
7, and then for
Usually [average] 2 years after you start your period, it will become regular. You will be able to predict when your period will start, how heavy/light it will be, when it will end, and how long it will last.