The longest normal time between periods is 35 days. Although it is abnormal for periods to occur more than 35 days apart, there are exceptions when this is considered normal.
The average time span between periods is 28 days from the first day of the last period to the first day of the next period. The shortest normal time span between periods is 21 days. It is abnormal for a period to occur two weeks or less from the previous period. Very athletic females who participate in intense sports may experience periods that occur far apart. This is normal.
Irregular and inconsistent periods may occur during the first several years of menstruation. This is normal for young females. Girls who are under age 11 when their periods start should see a gynecologist. Girls who are over 16 and have not started their periods yet should seek medical attention.