For teens, irregular or long cycles are common for up to six years after their first period as hormones balance out. Common Causes of Extended Bleeding Menstrual cycle: What's normal, what's not - Mayo Clinic
A "long period" can mean two different things: bleeding that lasts for many days or a cycle where the time between periods is unusually long. While a typical period lasts 3–7 days within a 21–35 day cycle, significant deviations can be your body's way of signaling a change in health or lifestyle. Understanding "Long" Periods Period
A cycle that lasts longer than 35 to 40 days before the next period begins. For teens, irregular or long cycles are common