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 5 days.
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.