Some lucky women have a precise 28 day cycle, but the usual variation is from 25 to 33 days. The actual length is not an indication that something is wrong, but variation from what is normal for that person can suggest that careful monitoring is required.
Hormonal imbalance is assumed to be the cause.
It can get so bad for some that the suffer period pain for up to 15 days of the cycle. There are two types of period pain. Spasmodic pain is where there is cramping in the lower abdomen which begins at the start of the period and can last up to seven days. Congestive pain is a dull ache before the period and stops at the start of the period. This is where there is no period, after a regular cycle has been established.