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.
Helpful Tools: Menstrual Cycle Lengths Chart provides a comprehensive graph of your cycles and period lengths, showing if any deviate from average cycle length. Cycle Lengths and Period Lengths reports group together cycles and periods by shortest or longest lengths.