A normal period starts (menarche) between the ages of 9 and 17, and comes every 21 to 40 days. Only 15% of women actually get periods exactly every 28 days.
There is a wide range of normal, and consistency within your own cycle is most important. If you always get a period every 24 days, for years and years, and then all of a sudden they start to come every 38 days, this is a significant change for you, and you should bring this to the attention of your gyn. If however, you always get a period every 38 days, then this is normal for you.
On average, normal menses last between 3 and 8 days, and should not be excessively heavy.