Break through bleeding could last from 1-3 months,it takes about that for your body to adjust to the birth control.Depends on what kind you take as well,if it's the kind that you have less periods,could take longer.Your body already knows to have your mentrual cycle natrually,when you take birth control it will fight that process.It's very normal to have break through bleeding,unless it goes beyond 3 months.Everyone is different,you can do research on your birth control to find out how others adjusted to it.