On an average, a girl would have her first period at the age of 12 or 13. However, the age of the menstruation may differ on the basis of hereditary factors and lifestyle. Some girls hit their first cycle at the age of 10 and some don't hit it till they are 15. So, this differentiation is subjective.
In case, you have been wondering, relax! Mostly, mother and daughter get their first period at the same age. To answer the question of "How long do periods last?", well, that varies from case to case, as every girl/woman has a different body constitution. Further explanation about the same is provided here.
Under Normal Circumstances
Under normal circumstances and in most cases, periods last for around 3 to 6 days. Nonetheless, the first period may last for a maximum of 10 days. This is due to the fact that it is the first period and the cycle needs to get in order. However, once the cycle regularizes, you will be able to ascertain your period span.
Some women have very light periods, they last for only a couple of days. However, normally, they would last for a minimum of 3 days and a maximum of 6 days. An additional 7th day is also normal.
In case you have spotting only for a day, it is not your period and could be due to any other reason. So, get it checked out with your gynecologist. Post the first 3-4 cycles, the periods should not last for more than 7 days. So, in case this length is persistent, visit a gynecologist, it could be nothing, but it is better to be on the safe side.
When on Birth Control
Many women who are planning on "going on the pill" mostly have no knowledge about how long their periods last when on birth control. If this is your question, then I have many answers for you. Different birth control pills have different effects on the menstrual cycle.
Nonetheless, the most appropriate birth control would be one that would not cause any major changes in your cycle. Most birth control pills cause the cycle to be that of 28 days. This is normal, any other aberrations needs to be discussed with your gynecologist.
On an average, on birth control, the period should last for a minimum of 2 days to 9 days. Some women face 12 days of periods, if this is discussed with the gynecologist, it is fine. Else, your gynecologist and I would both suggest you to consider another method.
Many women face irregular periods. This could have many reasons, hence, they need to get it checked out by a gynecologist. Now let's come to the question, "how long do irregular periods last?". To start off with, most teen face irregular periods. This is because their cycle is still not completely set.
As such, the time span may vary slightly, from 3 to 10 days. However, post teen age, mostly, the cycle regularizes to quite an extent. It may seem irregular if the cycle is for anything other than 30 days, like 27 days or 32 days.
So, try to calculate from the start of your one period, to the start of the other. If this length stays almost constant, you are regular. If not, visit a gynecologist and try to figure out the reason for the irregularity.
Even if the periods are irregular, their time span should be within the 4-12 days range. This could be deciphered on the same grounds as regular periods. However, in spite of the fact that the periods are irregular, the bleeding and the length should be relatively regular.
So in case you have your periods for 4-6 days, then whenever you get your periods, that is how long they should last. Too much variation in the length should be discussed with the gynecologist.
Periods and the menstrual cycle may seem like a curse at times. However, they are often indicators of smooth bodily functioning, for women. So, keep keen track over your cycle and take care.
Disclaimer: This is for informative purposes only, and should not be used as a replacement for expert medical advice.