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It is different for different women. Some women have periods every 30 days, and some have them every two weeks, and some get them for months on end (a friend of mine) but only in very small amounts (she was not pregnant, either).

So, I always say, never "guess" what your cycle is. Chances are, if you're a teen, your cycle is still fluctuating, so it's not going to be easy to track unless you force your cycle to adhere to a birth control pill regimen (which is basically hormones that keep you from bleeding until your fourth week in the month).

That count would usually be from day you begin to day you begin again. One full cycle.

They don't tell you in health class because most schools suck royally when it comes to health classes.

As for your doctor, I don't think a doctor ever told me that either. I just did routine checkups until I got to an age where I needed a gynecologist. By that point, I suppose that it was assumed that I would already know all about my period.

The reason you don't count from the end of your period is because that's extremely iffy. Most women will "finish" they periods, but still have some spotting for a day or two. So you can start counting once it starts to slow, or once it's totally 100% over, or what? Just too much confusion. Whereas the begining is easy. When there's blood, start counting.

If you see a gynecologist, they should always ask you how long it has been since your last period started.

That's not normal. If this were your natural cycle, I am sure a gynecologist would want to run some tests on you to make sure everything is ok.

Severe weight loss can cause you to miss your period. So many anorexics find that they stop menstruating. The reason is that the body recognizes that it is not healthy enough to support a fetus, so the reproductive systems shut down. Beyond the obvious nutritionary problems, this can cause a lot of issues as well.