Is it normal to be on your period for 12 days? I'm only 15 and I'm nervous. When will it go off?

There is practically no normal when it comes to anything to do with menstruation. Many women have much longer periods when in the teens and 20s, with more volume too, than they do as they age.
That being said, 12 days is a long time, especially if the flow isn't tapering off. How often do you change a tampon or pad on day 12?
You could have a condition that should be looked at, even if your doctor says that at your age, you don't need testing unless it persists month after month.
Most such conditions have to do with the uterus. But rarely they can be connected to hormones such as thyroid. There's no way to guess now.

Do you have a mom or close family female family member, It is so important to be able to talk and communicate about these types of issues.

The questions asked by the first expert are important, since the level of the cycle?